1) Physics playground / wrapper
either a physics playground in your favourite api, or if you're really cool make a wrapper by subclassing ICollisionManager, ITriangleSelector and all that jazz.
2) multiplayer network game
a fast action game with more than one player, so you'll have to learn/use a network api properly. the game isn't important and can be a single screener (pong? asteroids?).. what is important is trying to make it responsive.
okay I know there were other suggestions that should perhaps have made it here.. if anyone is upset that they were not included, then cry below and i'll edit the poll.

oh and please only vote if you're going to take part, the idea is to try and suss out what people would prefer to take part in.