One of my favorite things in Ponyo is Sosuke's toy boat which is powered by steam. This was a toy that Hayao Miyazaki had as a child, and he reintroduced it in his film. Are you curious to know how it works? Would you like to make one of these boats yourself?
This site explains the thermodynamics behind the steam-powered boat. It's very simple science, really. and requires no moving parts or gears.
SciToys also sells toys like the steam boat. This second photo is an item for sale, and runs on candle power, just like Sosuke's toy boat. You can visit the SciToys Catalog and and look around. If you're a science fan, these toys will really grab your attention (and probably your wallet).