A popular theme that works well for either a boy or a girl is a clown or other circus_related cake. They are usually very bright and colorful, which makes for instant attraction and a great focal point for a party table.
Photo Cakes: Run out of birthday cake ideas? Making a cake with a translucent, sugar "photo" is easy, and enjoyable by birthday boys and girls of all ages. Just take a favorite photo to your local cake decorating store, and they can prepare an edible version of it for a few dollars. Follow the instructions provided, and you can add it to the top of a simple, iced cake. Voila! Instant birthday gratification!