Build yourself a commuti-uni. This is a unicycle with a large diameter wheel (typically 26") and short cranks (5"). With practice, you can go quite fast, yet you have a machine that is much simpler and smaller than a bicycle. Start with a unicycle that has a 24" wheel. Most have clearance for the 26." You can buy used unicycles at garage sales for $20- $40. You'll need to buy a rim and spoke it on yourself - or have a bike shop do it. Ideally, it will be an inexpensive unicycle with cottered cranks, so you can get the shorter cranks for it fairl easily. Be careful, it is hard to stop quickly on commuti-unis.
BikeTune, Have you ever heard of an "ultimate wheel"? I saw some guy in a suit on this unicycle type thing, but it had no seat post! I asked him about it, and he said its an "ultimate wheel". He asked if i wanted to try, but i didnt have the time to make a fool outta myself anyways, he too was commuting on this, im sure i was just one of hundreds who asked him about it that day.
Why not just buy one? Unicyclesource.com has 26" 28" and 36" wheel unicycles.
I've never tried anyof them but a 36" wheel sounds like fun!!