If you want something more challenging than a unicycle, you should try the BC wheel (the name comes from the stone wheel that characters in the popular comic strip ride). It's basically a non motorized version of what they describe in the article. Just a free-spinning wheel with metal plates you stand in. The other name it goes by is an "impossible wheel". They are quite difficult to ride - but some can ride them and do tricks similar to skateboarding. Here's one of the best guys at riding them: