Free Printable. Ask the same 20 questions year after year (1-18, but probably age 3 or 4 is a great age to start) and see how your child's responses change year to year. Makes a great keepsake to give to them when they turn 18.
Homemade pet jelly fish. that's about the neatest thing EVER. I did this with my 3rd, 4th and 5th grade class. Told them to bring in a bottle and a bag. Some of the kids brought it in and the rest of the kids thought it was dumb. After we did this project, the kids who thought it was dumb, didn't think so anymore. Kids who made them, named them and love them at home now.