The Car That Watches Out for Its Head
Here's a simple but cool model for kids. The creator's description follows:
"This small robot starts with raising its head a little up and keeping it raised because gravity pulls it down. It goes direct until it approaches any thing then it decelerates its speed (so that if it hits that thing, the hit is not hard). Then when it is 25cm far from that thing, it raises its head and turns back randomly either to the right or to the left. The randomness was important to get out from small spaces after many tries."