Earth Day craft instructions for "Earth Day Coffee Filter with Hands Craft 2" from easypreschoolcraft.blogspot.com. Very cute idea but requires a coffee filter to complete as described. A modification would be to have them tear green construction paper and glue onto blue construction paper, print off a globe template and let them color it in, or let them color their own globe from scratch.
Backyard Water Rocket With a catch & release mechanism, you can get higher pressure--with significantly higher flights! This version will be safer, especially with small kids around. (Popular Mechanics)