[Mike Schreiber]

Tallest news paper, electrical tape, popsicle stick, straw tower holding a 1 kg mass is fun, I've seen them get up to 8ft (height limited by what they could reach standing on a chair)

As mentioned before mouse trap car is always a cool one. There's a bunch of variants on it. It's a good way to transition into an engineering lesson on wheel size and acceleration too.

Kettering University does these types of things weekly with students during lunch and gives prizes, they post most of them online, but I can't seem to find them, I'll edit if I come across any particularly cool ones.