Rolling Ball Systems are a lot of fun. You stick balls in the top and they make lots of noise and motion until they reach the bottom. Mechanized ones automatically move the balls from the bottom to the top so you can get a continuous cacophany which is always fun.
Here is a picture of the Frigits, which is the toy that initially got me interested in RBSs.
Next is one of the two K*Nex sets that I bought. This was fun, but it only had movement. You need more noise in order to be fun! Also, it took too long to build. It was still fun though!
Now here's the wonderful Chaos World of Motion set. I bought 3 sets and put them together. It makes a lot of noise and, if you have enough marbles, the motorized system keeps the marbles moving around continuously.