Tennis balls, stress balls, Slinkies are some of my favorites. Also, those silicone wrist rests for use with computer mice are lots of fun.
Quote:
When I was around 12 I made a wind turbine of my own design and a working clock complete with pendulum and counterweights out of K'nex.
Aaand just like that you put my K'nex suspension bridge to shame!

I used to construct Lego baseball stadiums.