those things where you can put a penny on a track to set it up, and it goes around and around and around and around until plink, down through the center hole.
There's one at third planet in the mall, and there's one in the entryway to our science museum exploration place.
here's the best thing...
1) get a pile of pennies
2) go to exploration place on a pretty crowded day
3) hand a penny to a random kid standing by the coin well
4) hand another penny to another random kid
5) that's all it takes, kids notice and pull in and you stand aside and they're coming to you for pennies until you are out, and they're surprisingly understanding if you announce that the end is near. it takes a while to run out. usually the reason I stop handing them out isn't supply, it's some other poor mom is trying to get her kid out of there and here's this crazy lady handing all the kids pennies and they're watching them all spin around and around and it's not helping, so I pretend say I'm out, then start back up again when they're gone.
all the kids are excited and the pennies are zooming around and they're happy and you're happy.
something about centrifugal forces or momentum or science.
the best things in life aren't always free, sometimes they're 20 or 30 cents.