Just wanted to share this with you... I found this hummer ride on toy online:http://www.shop.com/op/~Little_Tikes_Hummer_Black-prod-36218636-48525441?sourceid=298
The thing is 4 feet long, and has its own power source. It's obviously too weak for you to ride on, but if it can carry two kids, it can probably carry a bigger battery, a laptop, and then some more equipment. It uses 12V power and should be pretty easy to set-up... Would basically need a way to control the steering, and this could make a good robot base. It has 14" wheels and can go up to 5 MPH.
Update: the maximum weight capacity is actually rated at a whole 130 lbs and it has storage space behind the seats... I'm starting to seriously consider this for my robot because of its hugely practical aspect. The only downside is that it would look a bit silly