This video shows how I got a two wheel model to balance using a home made gyroscope from a toy motor and 4 stacked cds for the rotor. There is a little trick to doing this that I talk about in the video. Hope others will make one too and improve on it.
I know this has been done many times before but I couldn't find the details on how to do.
I also tried attaching two parallel wheels but it just wants to spin in a circle and when I try to move it in the direction of the wheels it falls over easily unless I push very slowly. I suppose motorized wheels will work but without some active dynamic balancing mechanism it would be pretty susceptible to falling.