Think about it.
Old and young alike love cars, boats, airplanes.
Cars and race cars are very popular themes for birthdays and other parties.
Here are some cakes to check out to use with your theme party that uses some mode of transportation:
Can't make up your mind? You could always do a scene with a combination - a train, next to water with a boat, next to a freeway with cars, and then above it all an airplane.
Hope these photos linked above give you inspiration!