One of the group activities that is really effective in my AP class is when they teach each other Multiple Choice questions. They split up and do 3 to 4 problems in groups (and there are 4 groups of problems) and then get shuffled so that each new group has one person who has done all the other questions. I think that they get a lot out of explaining to each other and get to see more problems than they could do on their own in the same time.
I've tried this with Free Response problems, and I don't think it's nearly as effective (as some people can't really e en do them, so the they can't explain them!).