Found in the MAKE Flickr Pool: This sweet whip by Peter Wagner of Davis, CA, packs an unaltered unicycle for the back wheel and seat, with a bolt-on front wheel if you need two.

Famous Davis mayor, conservationist, artist and all-around thoughtful person, dear Julie Partansky (a twin like Myself!) gave me her old unicycle. Which in 2000 I adapted into being able to convert back and forth from unicycle to mini Hi Wheel Safety Bicycle!
2 bolts and a quick release will do it! 2 minutes, tops… Tal, my neighbor, calls it the Wheely Bike. It’s easy to go slow and spin the handle bars, circus & kunstrad style, too!