ThinkProgress and TreeHugger have both shared a brilliant idea: When credit card companies send you junk mail, send it back.
I get all my junk mail at my permanent address, so I can't do it at school, but it sounds like a great idea to me. To quote TreeHugger:
If enough people take a second to participate, this could irk the mailroom staff at JP Morgan et al enough to spur the sort of conversation [...]. Or better yet, it could provide the media -- ever hungry for the next 'Occupy Wall Street' angle -- with fodder for a new narrative.
TreeHugger recommends including a note with exactly why you're angry at the bank, but I've heard some other ideas - for example, my co-workers fold up all of the paper that came with the return envelope and just send it all back, sparing themselves having to throw any of it away.
So if you want some easy activism AND to get rid of some junk mail, send back those credit card offers!