To jazz it up, I made red and white zebra cake for the layers. It's pretty similar in concept to rainbow cake in that you pour small amounts of batter directly on top of each other, forming concentric circles. In the rainbow cake, although the batter didn't fill the pan, it evened out during baking and filled up the round pan perfectly. Unfortunately, that doesn't work as well with a rectangular pan, so the layers were seriously tilted. Whoops! I evened them out and added a plain vanilla middle layer, so it was nice and flat in the end.
For the decorations, I made a big Muno out of red fondant. Bug Bug really wanted me to do the background like Muno's house, but I didn't like how the random fondant "rocks" looked, so at 11 pm, I pulled them off the cake, refrosted the top, and went back to my original plan: a bright yellow background with a wavy red frame around it. MUCH better. It came out super cute and the boy liked it very much. It tasted good, too!
These are the cake and frosting recipes I used: silver white cake and basic bakery buttercream frosting. With three thin layers, a 9x13" cake was big enough to serve at least 25 people and still have a third of it left over.
Happy birthday, boy-o!