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.
Cakes are usually associated with sweetness, especially among children. Kids love sweet things and that is why they always see a birthday or any special event as the time to enjoy. For your birthday cake, you could choose chocolate cake. Children and adult would love that. You could also prepare cake that is not too sweet in case of adult guests who are not too interested in sweet things or who are diabetic. Having this combination ensures that everyone has a nice time at your birthday party.