Check out this video of a small humanoid robot riding a two-wheeled bike. No training wheels! I couldn't do this until I was about 5 or 6 years old. It won't be long now folks: there will be little robots dashing about all over the place.
It's a lovely piece of work all around. There is something charming about this one. When we ride a bike we are forced to move as the bike's mechanism dictates. The movements of this particular robot end up looking just like a human because the machine is on...well...another machine. It, like us, moves as necessary to operate the bicycle.
I can't tell you much more about this little bike-riding robot, because the site is in Japanese. The YouTube description says it was created by a Dr.Guero. Here is a link to the official web site for the miniature robot-riding robot.
If you are interested in small humanoid robots, you might check out Tomy I-SOBOT Robot -- a very capable (and funny!) little robot that many people are using as a platform for their own experiments.