My friend Denise mixes cake crumbs from white cake into her buttercream icing and a little ivory color to make the buttercream look like sand...and then ices the cake with that. It's from the "cake Spackle" idea that Toba Garret uses on her cakes under her icing. Not quite so much cake in the icing though. It looks good and doesn't give any strange taste or texture to the cake like adding unbleached sugar, brown sugar or grahmcracker crumbs would. You'll just have some crumbs in the icing...oh gee that might even make decorating easier since you don't have to worry about crumb coating! Just a thought. I would type sandcastle cake into Google images and look at the pictures for different ideas. Add a few white chocolate molded seashells and Voila