This post contains affiliate links. If you buy something through these links, I may earn a small commission at no extra cost to you. I only recommend things I’d actually use.
The first time I saw a One-der the Sea birthday party on Pinterest, I stopped scrolling. Just stopped. Teal and coral and that weird gorgeous iridescent shimmer — it looked like someone had bottled actual ocean light and turned it into a party.
It’s one of the most photogenic themes going right now. And it works just as well for a baby turning one as it does for a seven-year-old who’s been obsessed with The Little Mermaid since Tuesday. The pun is perfect for a first birthday, but the mermaid vibe? Completely ageless.
Here’s how to actually pull it off — including the one $6 craft store hack that makes everything look twice as expensive.
Start With the Color Palette (This Is Where the Magic Is)
The whole vibe lives and dies here. You want: deep teal, coral pink, sea green, soft lavender, and anything iridescent or holographic. Gold accents. White. That pale, shimmery aqua that makes you think of sunlight hitting shallow water.
What you don’t want: navy. Navy turns the whole thing into nautical instead of mermaid. There’s a difference.
Pick two hero colors — teal and coral is the classic combination — and use iridescent as your accent. It doesn’t compete with either color. It just makes everything look more magical, which is exactly the point.
The Iridescent Balloon Arch (The Photo Moment)
This is the element that gets reshared most. If you do nothing else elaborate, do this.
You need:
- Teal balloons (11-inch and 5-inch both)
- Coral or blush pink balloons
- Iridescent or pearl-finish clear balloons — the shimmer ones, not regular clear
- A balloon arch strip or command hooks to anchor it
The trick is mixing the sizes. Big teal, medium coral, small iridescent stuffed between them wherever there’s a gap. The iridescent ones fill in without adding visual weight, which is why the whole arch looks airy and floaty instead of heavy.
You don’t need a professional balloon artist for this. The arch strip does the work — just inflate, tie, and push the balloon tail through the hole. Thirty minutes, maybe. I’ve seen 10-minute tutorials on it. Hang it as a backdrop behind the high chair or cake table and every single photo taken near it will look amazing.
iridescent balloon arch kit — get a pack that includes the strip and some balloons together, it’s cheaper than buying separately.
The Dollar Tree Seashell Scatter
Go to Dollar Tree. Buy every bag of decorative seashells and starfish they have. This is not a joke. They usually stock them in the wedding and craft sections, and they’re genuinely pretty — not cheap-looking.
Spread them across the tables in clusters. Add some pearl-finish faux gems scattered between them. Tuck in some blue or teal crinkle paper as a base if you want the “ocean floor” look.
Cost for a 6-foot table: probably $8. That’s it. The shells and the pearl gems together look like you spent forty dollars at a party supply store.
One thing to get at the craft store instead of Dollar Tree: the iridescent fish netting that sometimes comes in rolls. Dollar Tree carries it seasonally. If you find it, grab two. If not, craft stores have it for a few dollars and you drape it over the table edge or pin it behind the backdrop. Instant underwater.
The Iridescent Paper Hack (Do This for Everything)
Here’s the thing I wish someone had told me earlier: iridescent and holographic paper from the craft store — the kind that’s like $4–$6 for a pack — is the cheapest way to make a party look genuinely mermaid-magical.
You use it for:
The banner. Cut out letters or shapes, punch holes, string them on twine. “ONE-DER THE SEA” in iridescent paper with a couple of shell cutouts between the letters looks like something from Etsy. It’s not.
Favor bags. Cut iridescent paper into rectangles, fold and tape into simple bags, add a ribbon handle. Or just buy plain white bags and add a rectangle of iridescent paper to the front like a label. Either way it looks intentional and special.
Table accents. Cut fish tails, starfish, or wave shapes. Scatter them among the seashells. Tuck one under each cupcake on the dessert table. They catch light in a way that regular colored paper just doesn’t.
Gift tags. Cut small circles or tags from it. Much more special than the printed ones from the store.
The total cost for all of this is probably $8–$12. The result looks like a Pinterest board came to life.
The “One-der the Sea” Invitation
For a DIY version: find a cream or white card with a coral or teal border, add a printed shell or pearl motif in the center, and write (or print) the party details in a soft gold or teal script font.
Elements to include:
- A seashell or pearl cluster at the top
- “You’re invited to a ONE-DER THE SEA celebration” as the headline
- Baby’s name in a larger font
- Date, time, location
- RSVP details
If you’re designing digitally, Canva has free templates that are easy to customize with your own colors and fonts. Search “mermaid first birthday invitation” and you’ll have a dozen starting points.
For printed invites, 21 Super Profitable Printable Invitations That Sell Fast is actually a good place to find sellers on Etsy who do custom versions for under $10.
The Mermaid Tail Cake
This is the centerpiece and worth doing well. The classic version is a two-tier cake — pale teal or seafoam frosting with textured scales (a spoon creates them, or a simple palette knife) and a fondant mermaid tail curving up from the top.
If you’re ordering from a bakery, here’s what to tell them:
- Seafoam or pale teal buttercream, textured not smooth
- Simple scale pattern — a spoon-pressed scallop all over is fine
- A fondant tail in teal and coral with some pearl luster dust on it
- “ONE” written in gold or coral on the front
That’s it. You don’t need to show them a picture from a wedding cake Instagram. Simple instructions produce good results.
If you’re making it at home: a boxed cake works fine. The decoration is what makes it. Mermaid tail cake toppers run about $8–$12 and they look great — you don’t have to make fondant anything.
Ocean-Themed Food Names (The Free Upgrade)
This costs nothing and makes the food table look intentional and curated. Just rename things.
- Sandy Shore Rice Krispie Treats — coat them in crushed golden Oreos or graham cracker crumbs before the marshmallow sets. Press a shell into the top.
- Sea Foam Punch — lemonade or limeade with sherbet and ginger ale. It froths beautifully and the color is perfect.
- Crab Cake Bites — any small red or orange-frosted cupcake or cake pop with crab legs made from pretzel sticks.
- Mermaid Bark — white chocolate melted and swirled with teal and pink, poured flat, topped with sprinkles and shells, then broken into pieces.
- Pearl of the Ocean Cake Pops — any flavor, dipped in iridescent or white chocolate with a pearl sprinkle on top.
- Under the Sea Jello Cups — blue Jello with Swedish fish inside. Kids lose their minds over these.
Make tent cards for each one using — you guessed it — small rectangles of iridescent paper.
Mermaid Crown Craft Station
This is the activity that photographs best and keeps kids busy for a solid 15–20 minutes.
Set out:
- Pre-cut cardstock crown bases (one per guest — you can cut these from gold or iridescent cardstock ahead of time)
- Foam stickers in shell, star, and pearl shapes
- Stick-on gems in teal, coral, and purple
- Glue sticks
- Ribbon or elastic to attach at the back
Hot glue a piece of elastic to each crown before guests arrive. Kids decorate, you tie or staple the elastic to fit, and they wear their crowns for the rest of the party.
foam shell stickers for crafts — get a multipack, you’ll go through them.
For adults who want to make one too: let them. That’s the best sign that the craft station is good.
Shell Favor Bags with Pearl Candy
Fill small bags — iridescent DIY ones or clear iridescent favor bags — with:
- Pearl candy (the round white sugar-coated ones)
- A few shell-shaped chocolates or gummies
- A small starfish tag that says “Thank you for helping us celebrate our little pearl!”
Tie with teal ribbon. Done.
You can find pearl candy in bulk online or at baking supply stores. Pearl candy bulk — $8–10 for enough to fill 20 bags.
The whole favor costs maybe $1–2 per guest. It looks like it cost five.
What to Skip
Two things come up in mermaid party searches that I’d skip: the sand dollar cookies that require actual artistic skill to decorate, and the “fish bowl” centerpieces with real fish. The cookies are a time trap if you’re not already good at royal icing. The fish bowls are a logistical mess and you end up responsible for fish afterward.
The Dollar Tree shells and iridescent paper will do more for your photos than both of those combined.
Frequently Asked Questions
Does the One-der the Sea theme only work for a first birthday? No. Drop the “one-der” pun for older birthdays and just lean into the mermaid/under the sea theme. The colors and decorations are exactly the same. Works through elementary school age without changing anything except the signage.
How do I keep the balloon arch from falling? Command hooks rated for 5+ lbs and the arch strip system. Anchor both ends first, then fill in the middle. Don’t hang it somewhere with direct sunlight or it’ll pop.
Where’s the cheapest place to buy iridescent paper? Michael’s or Hobby Lobby with a coupon, or Amazon in a multipack. Look for “holographic cardstock” or “iridescent shimmer paper” — either works.
Can I make the mermaid crown craft work for really young guests (toddlers)? Sort of. Swap loose gems and stickers for larger foam pieces they can press on without help, and pre-decorate half the crown so they’re just adding a few pieces. The crown itself is what they want to wear, not the craft.
What’s a good alternative to the balloon arch if I’m doing this at home with low ceilings? A balloon garland hung horizontally across a wall, or just a cluster of balloons tied to the high chair. The arch is great but it’s not worth fighting your ceiling over.
For more party planning ideas that work even when you’re short on time, the 25 Holiday Party Games That Work for Any Celebration list has some good activity options that translate well to kids’ parties. And if you’re looking at the DIY favor bag idea and thinking it sounds like a craft you’d actually enjoy making regularly, 7 Crafty Hobbies That Are Surprisingly Relaxing might be worth reading after this.
One-der the Sea is one of those themes that looks like it cost double what it actually did. That’s the sweet spot for a first birthday — beautiful photos, manageable budget, and an arch that the guests will be standing in front of for the entire party.
You’ve been thinking about it for three weeks. The park party idea keeps coming back. It&rsquo
Skip the boring brunch routine and try these 18 Mother's Day games for adults that bring out laughs,
This post contains affiliate links. If you buy something through these links, I may earn a small com
Leave a Reply