Other than using our computers to play games the number one thing kids do is look for photos, be it for school projects or their own interest.
I love all the options at BigHugeLabs. Make a magazine cover, movie poster, pocket photo album, etc. There are months worth of activities for kids right there. Create a literary or news magazine and add designing the cover as part of it. Ditto for a movie poster. The possibilities are endless as long as you have interested kids and access to a couple of computers with internet. I can't wait to present some of these options to our fledgling Tween TAB. (I need a better name for that.)
A couple of other image sites I love are:
1. Picnik - my favorite tool for doing simple adjustments to photos, though there are some more advanced options as well
2. BlockPoster - I made some photo backdrops, so kids could pretend they were at the beach.
(I'm having way too much fun making librarian trading cards for some of my colleagues.)