Stirlings are an awesome technology that has stumped inventors for well over 200 years. People just couldn't seem to get one side hot enough with the other side cold enough to do usable work while keeping the two halves insulated from each other. I think you'll like seeing all the YouTube examples of the challenge. Being in the HVAC field, I would have thought you would have known about a machine which has the potential (unproven yet) to be the best heat pump if ran backwards.
HHO is also pretty fun to play with. It makes a very nice torch flame which opens up lots of opportunities. You just have to love something that powerful that has no toxic exhaust.
Plasma, now that's a truly intriguing substance. Nikola Tesla used to give demonstrations of holding a ball of plasma in his bare hands in from of an audience of electrical engineers. Still today, we can't figure out exactly how, but we've termed the phenomenon "ball lightning".
You might also enjoy the videos of 80-100K Joule railguns where the projectile is non-conducting and propelled by a conductive plasma sabot. That one almost seems to defy a few laws.