In reply to robbo157:
I looked into this briefly a while ago before a trip - Google turned out a few options, one of which was something like $40p/d, one $60 -100p/d. Can't remember the names off-hand, but Google should have 'em.
My thought was to head north to Estes Park, then up Trail Ridge Road. Didn't rent in the end, which is probably just as well - I'd forgotten that you start at something like 2000m, even before you get to the uphills. Just climbing stairs left me out of breath for a few days