To go with MFW:ECC Castles @ Germany How to build an easy catapult for kids and learn about tension. Build a simple LEGO catapult with basic bricks. Fun STEM activity for kindergarten and grade school kids.
Marble run made out of toilet paper rolls and paper plates. Ok, so this was a rad idea but pretty tough to do with 2 kids. It would have been good with just my 5 year old. The kids got frustrated that it took so long. I ended up finishing it with a Martin