With the matrix working properly the question of the appropriate games arises. My previous experiments in matrix gaming took place on a 8x8 matrix and I've programmed some kind of a racing game, Space Invaders and the obvious Pong. Furthermore Convay's Game of Life but I want something interactive.
Some more games that come to mind are:
Tetris (or variations of the theme like Dr. Mario)
Some kind of Jump'n'run
Somethin like a shooting gallery
One thing which was clear is that I want to use the Nunchuk controller of a Nintendo Wii since it works rather well and effortlessly on an Arduino and I don't have to put somethin prototype-looking together myself and hand it to my wife.
The idea is that my wife will have to beat the game(s) running on the cake and if she does it, a Servo hidden in the base of the cake will open move out her present (or a hint where I've hidden the present since it most likely won't fit into the base).
I will post more as soon as I try out some of the game ideas...that should happen this month.