An extravagant cake is one way to mark a birthday, and using the cake to accentuate your child's interests can be a cool surprise for him. If he is a swimmer, he may get a splash out of a swimming pool-themed cake. Consult a bakery and see what design options are available to you. Alternatively, you can draw up your own and have it made -- or make it yourself, if you're up for it.
Turn a simple rectangular sheet cake into an Olympic swimming pool by icing the edges with white and the top with pool-water blue. For extra detail, build up the edges or indent the top an inch from the edge to simulate the space between the water and the edge of the pool. Then, use another contrasting colour to make the pool's lane guides. Use biscuits or wafers to make the starting blocks and additional icing for the arms and heads of swimmers.
Above-ground oasis cake
Your double or triple-layer round cake can be an above-ground swimming pool. Ice the side of the cake with white, or use chocolate bars or wafers to simulate the pool wall, then ice the top with bright blue icing. Use gumballs and jawbreakers for beach balls (or make them from icing), and iced pineapple rings for inner tubes. If it's a really large cake, you could also use doughnuts for the inner tubes.
Back garden model cake
Create a swimming pool back garden scene with a rectangular cake. Ice it with bright blue icing, then use white to trace the edge of the pool. You can use more white to simulate a concrete deck, or simply ice the rest with green to symbolise grass. Add narrow rectangles in contrasting colours on the water, to represent rafts, and on the deck, to represent towels. Use a chocolate bar for a diving board and gummy ring sweets for inner tubes.