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.
Yeah, but it was always upside down! lol