depends on your interests. My faves are media and non-religion which you cant do!! Unless you know how to manipulate the system that is!!
Seriously, it depends on a couple of things:
your interests eg music, art etc
and
how seriously you want to do this ie are you looking for the quick and easy answer/task or something you will enjoy doing
There is a thread already on this type of discussion further down the page which list a couple of suggestions
http://www.boredofstudies.org/community/showthread.php?t=15749 or you can brainstorm ideas here!!