i was thinking of doing this also, but i was told that it's not a convention and it's part of their jobs to write rec letters... but if i get in somewhere i'll probably go with it. i think something small is sufficient, or even a simple thank you letter. so chocolate/nice coffee is a great idea. just make sure its something they like (i mean if you never seen them drinking coffee.. i wouldn't get coffee). most people like chocolate. i think for female professors its always good to get flowers. i have one prof who loves ethnic decoration things (he has things from different countries all over his office) so i am considering getting him something small like a candle holder or decorative pen from pakistan.
so there's another idea for you- see what kinda decorations they have in their office and get them something that they could keep in their room.