I learned to ride a unicycle as a kid. I got rid of it a some point in a move, but at such time as I have more space, I'd like to get another one. Maybe with a 24 or 26" wheel, for a little more speed. Actually, it's useful kind of like one of the small folding bikes in that you can ride it somewhere and then just toss it in a bag and get on the train or toss it in someone's trunk if you're meeting someone. It's not very fast (I had a 20" one before) but it's fun and still faster than walking somewhere.
A good place to go learn is a tennis court, because you have a long stretch of smooth pavement next to a tall fence that you an hang onto. It might be helpful to start in a hallway first if you have one because you can hang on with both hands at once, but failing that, a tennis court fence is a good place.