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.
