My son wanted "a beach" for his birthday, so here is the result of my Beach Birthday Cake Idea. I started by baking a standard Devil's Food chocolate cake. I iced it as soon as it was cool with cream cheese frosting that I made (it's 3 oz cream cheese, a little milk, and 2 c sifted confectioner's sugar per batch. Easy!!)
Tip #1: That first icing is a "crumb coat." It's white and it seals the moisture into the cake. And it makes it a LOT easier to decorate the cake. Do a crumb coat and leave the cake in the fridge over night.
For decorating, I whipped up a bunch more cream cheese frosting. I then cut it in half and colored one blue (ocean) and one tan (beach). Tan is (roughly) 3 drips yellow food coloring, 2 red, and 1 blue. "Ocean" is roughly 2:1 blue:green.
Tip #2: To get the color right, mix the coloring in a separate container. Put one drip of the coloring into some water in a white container... like a white coffee cup. This will allow you to get the color tone right so things are not "too blue" or "too red". It's just for tone (if it's not right, adjust your mixture and drip into clean water in the cup.) Saturation (how dark the color is) is determined by how much coloring mixture you put into the frosting... so add the coloring mixture one drip at a time and mix thoroughly to see the color!
I iced the cake so that the beach line was not straight and did not intersect the sides symmetrically. I then added "sand" -- Nilla Wafers that had been ground in the blender. I just pressed these in a little.
Final decorations are Swedish Fish, a drink umbrella, and a beach towel. The last was (appropriately) a piece of salt water taffy that I melted in the microwave (3 seconds!!!), molded/cut to shape, and cut "fringes" with a knife. I am sure you could use other materials like marzipan or white chocolate to do this, but I did not have any around. The surf board is a plastic decoration I bought online.
In case you're wondering, the spiral thing is a special kind of sand castle I build with my son. It's also taffy but, disappointingly, I could not get the Nilla "sand" to stick to it as well as I would have liked.
The final touch, I wrote my son's name in the sand at the water line using the back end of the handle of a spoon.