Bucky Balls are tiny little powerful magnets that look like little round pellets. They stick together and can form basically any 3D object that you want. They are awesome.
I had a chance to play with these things a few days ago, and they're additively fun. In the video on the site you can see someone making all sorts of impressive geometric shapes with them with ease, but you don't have to be so precise. You can mold these things into perfect cubes, sure, but you can also make big clumps and work on making a beautiful sculpture with them. Or you can pull them all apart and toss them on a table and watch them all snap together as they roll around.
For $25, it's a little steep, but if you're looking for something to play with while you sit at your desk or watch TV, you could do a lot worse. [BuckyBalls]