Cheap experiments and demonstrations for kids.
Also, high concentrations of CO2 are a bit corrosive, so don’t go breathing the air in the tank directly. If you have access to a merry-go-round you can talk about the coriolis effect, and make some kids throw up (that’s a twofer). Have the kids gather around the table, on the merry-go-round (while spinning), and try to roll the marble back and forth. So try this yourself before making kids do it (a little practice goes a long way). None of the other kids will want to try it afterward.