Yes, I kinda attempted to keep it vague. As of this moment, I'm transitioning into management (or so have been the last month), and I need something to keep me entertained...
Using C#/Java, maybe some front-end stuff would be ideal. Or possibly getting into Ruby/RoR.
Find something that you or someone in your life use--or would use if it just did XYZ. Work on that.
If you have a certain skill set that you're looking to hone, tell us about that and maybe someone could offer suggestions.