Port a program / library from one language to another. It's great fun. Make sure you know one language well, and the other not so well.<p>I'm having fun at the moment porting Flash and Javascript games to Objective C (sometimes using the Cocos2D framework, sometimes not)<p>Porting is great because sometimes it's mindless grunt work (just map functionX to function Y) and sometimes there's more important considerations. If I'm feeling lazy or tired I'll stub out the hardest stuff and come back to it later when I'm more alert.<p>Good fun!