these make great tire tubes (foam tires) : alot of different companies carry these, and you can even find them at home depot from time to time. they ahve smaller ones, great to hold bearings, diff balls etc...
similar to this (this had a threaded top
but on a side note to add to the funny of this thread, i guess it is a very multi-use product:
i guess if you bring your kids to the track... LOL