I like this idea, but would try to extend the water theme. Put plastic fish inside with a gift card to Bass Pro Shops! A gift card to the pool store or local water park would be fun too. I'm all about making the gift card connect.
My Artistic Side: Dolling up ordinary finds--This is what you do with all those old strange bottles you've aquired after cleaning out your parents or grandparents house, and you're too sentimental to throw them away.