You started with a tricycle, then you moved to a regular two-wheel bike. Learning to ride a unicycle seems like the next step, right? If you agree, then check out the Nimbus Ultimate Wheel ($90). It's a unique one-wheel
bike cycle with no seat, so it's especially challenging for your balance. Since it has no handlebars, you'll end up working your core and upper body much more than you would on a regular ol' bike. Practicing riding this bike could help you with tons of other sports and activities that require balance, such as snowboarding, yoga, dance, and surfing.
Want to see someone give it a try? Then
Man, this guy is really good (just ignore the pleather pants)!