I talked my daughter out of a "tree" themed birthday (for 5 y/o's!) and into a Mermaid Party. I don't usually make her cake, but since she chose to have her party at a play place, I figured I should come through with SOMETHING.
So I found the idea for a Sand Castle Cake.
It was actually really easy! And yes, I did use my degree in Civil Engineering to build a CAKE.
This basically shows the construction. 2 square cakes, with the corners cut off. I trimmed about 3 inches off the jelly rolls and stood them up in the corners. I used skewers to hold them in place, but I think if we were having the party at home (and thus no traveling in the car) it wouldn't have been necessary. I topped the jelly rolls with the cut-off bottoms of jumbo cupcakes.
I tinted some canned frosting and frosted the whole thing. The sand is made from graham crumbs. It was a little hard to get it to stick, but once you got some of it on, you could hold the cake without the frosting sticking to you.
It would have been okay just like that, but we topped it off with sugar cones (with little flags sticking out of the top) and melt & pour chocolate seashells.
Everyone at the party was impressed and the girls were even worried that I was serving them real sand!