If you want to go that route, I would recommend picking an assignment you are very interested in, and going all out on it. Even if you get A's on your assignments, they're still probably not that impressive until you can put a few months of extra work into them. Also, if you discuss it with your professor, you have the added potential of building a good relationship with him/her.
Looking back, I really wish I had done something like this because I was in the same boat as you - a lot of motivation to build something, but not knowing what to build.
Well I meant in addition to any non-school related projects I do. :P The tasks we are asked to perform here are a joke. I recall a problem set last year that went something like this: "You have a Door object that has a state which can be 'open' or 'closed'. Write a mutator method to change the state of the door." What the hell? That was about 2/3 of the way through the course too. I know its a first year course but come one ...
Looking back, I really wish I had done something like this because I was in the same boat as you - a lot of motivation to build something, but not knowing what to build.