Ideas for a kids' group community service project?

I'm organizing a community service activity for a group of 30-50 kids ages 3 through 12 and need ideas. The activity will be about 1 or 1.5 hours long. I don't want to collect money or supplies because that is more the parents doing it and the kids doing nothing - besides, what would we do at the actual activity? Some ideas so far are writing letters to cheer up chronically ill kids through a penpal program at our local children's hospital or tying fleece scarves to donate to local homeless shelters in the winter.