Comment by flowerlad

No one has mentioned GBC yet (Great Ball Contraption). GBC is a great hobby for adults, especially if you have kids. Bricklink is indispensable to find parts for GBC.

Here are some GBC examples:

https://www.youtube.com/watch?v=rWd3vgLaA_M

https://www.youtube.com/watch?v=A9_b6PzrqyQ

https://www.youtube.com/watch?v=tpBa59oOjVc

https://www.youtube.com/watch?v=1XqnTFSPG1I

See also: http://www.greatballcontraption.com/wiki/Main_Page