Given that a lot of people have Photoshop, and those that don't can easily install Gimp, I think a photo editing community would be fun.
Everyone is given the same image and then they edit it to try to make it all nifty. Then we vote on the who's final image is the best. And this is also supposed to the a learning exercise, so people have to share what they did.
Just one image a week.
Who would want to take part in this, if I set it up?