Probably vinegar and baking soda (or powder? I can never remember)
When I was eight my mom and dad got me a rocket for Christmas that you mixed vinegar and baking soda in to launch it. I could never figure out how to make it work, and given my propensity for being an accident prone kid, that might be a good thing...:)
Baking soda. Baking powder is baking soda plus an acidic salt so it fizzes with just water.
I had a little toy submarine when I was a kid that was fueled by baking powder. It would produce a bubble of CO2 which would lift the sub to the surface, be released to the air and then sink back to the bottom until enough CO2 was produced to lift it again.
Soda. It foams up and you can try to clear your drains with it.
The really dangerous one is chlorine bleach plus ammonia. Emits a toxic fume. Never use bleach to clean up a basement floor full of kitty pee.