The dessert table had the bulk of the decor. I hung paper lanterns from the patio umbrella and trees, and made a backdrop out of ruffled plastic tablecloths.
The table was laid with a caramel colored tablecloth, Squares of burlap layered as a runner, and dollar store seashells arranged by my 4 year old. :)
These are also part of the decor, I can't resist getting the biggest bang for my buck and most things at this party served a dual purpose. We put our sand table behind the treat table with pails and shovels. You can't have a beach bash without sand! This allowed the ruffle streamers to backdrop the sand play and the colors helped set the ambiance.
Of course, the food served as decor too. I used the Nordicware Castle Pan to make a sandcastle cake. I mixed a vanilla cake mix and a spice cake. Next time I will just stick with the vanilla. It got a bit darker than I'd anticipated in the bundt pan. It was perfect inside, however.
I invested in a starfish shaped chocolate mold to decorate the sandcastle cake and some of the cupcakes. I used peanut butter chips to fill the molds. I frosted all the cupcakes with a basic vanilla icing and then rolled them in crushed graham crackers. I tried to leave the very top of the starfish cupcakes free of crumbs to help the starfish stick. If you are going to transport them consider mounting them to toothpicks to help them stay in place. We did use 1/2 toothpicks on the other cupcakes to keep the gumballs in place. My kids had a lot of fun helping decorate these. I love that they are foolproof enough that the kids can help and I am still happy with the end result.