I looked into model steam engines a while back, but decided I had too many unfinished hobbies already, so I didn't dive in. Maybe one day...
Some of my favorite things are railroad museums and antique engine meetups. I'm also interested in more passive systems like Sterling engines, although I have yet to build one of those, too. It seems like a good way to create a low-powered air circulation fan or something with a similarly low starting torque.