So I decided to try making it stand upright...

It kind of works. Keeping it upright requires keeping it's speed low because if it's going faster than it can spin its wheels then it falls over. It also shakes a fair bit which I think is because of backlash both in where the wheels attach to the motors, and within the gears in the motors themselves. I also feel like there's something wrong with my balancing algorithm but it's kind of hard to tell with the loose wheels and relatively slow motors.