Unless things have changed since I went in January, you can rent them by the hour. We did so, and it was a lot of fun just riding around the camping loops as something to do for a few hours. I think it was $12/hour.
However, you have to rent them from the Meadows, so that means walking or taking an internal bus. Also, they close at 5 I think, so you would have to rent by 4:00.
You can't get a reservation just to rent one for an hour, so you may or may not be able to get one. But we just called before we left WL to see if they had any, and they told us to come on over.
Have fun!