I've been thinking about this Bicycle Blankenship. see http://www.youtube.com/watch?v=G8sFB-yL1c4
seems to be a fairly simple build. We usually carry two single speed bikes to camp with. Sometimes my disability leaves my balance off and I think this may be the perfect solution to riding and then having a problem.
He even has a book published for the build on Amazon. http://www.amazon.com/How-Built-Own-Four-Wheel-Bicycle/dp/1441411224/ref=sr_1_1?ie=UTF8&qid=1338903010&sr=8-1
Just need to go scrounge me up a smaller kids size bike I think.