It has a lot to do with what your comfortable doing.

^ What he said! Do whatever you want, really... something that won't put too much pressure on you. Sketches are good 'cause ya just sketch 'em and they're done! I think...

They can also vary. It's TOTALLY up to you! You can even just like, note someone and ask them what they'd like!

I'm sure anyone would be delighted to get art from someone like you. :D