Last year we saw an amazing bicycle that had been hacked so that it could change height on-the-fly. Like this:
I could not deduce how it worked, but Rachel over at Rako Bikes figured it out–and even made an Instructable showing you how to build your own. You’ll need a way to cut metal, access to a welder and a budget for some basic bike parts and tools–and, of course, a full-suspension mountain bike you’re willing to hack up–but she makes it look pretty do-able.
Check out the build instructions here.