Don't know much about car rentals, but we rented scooters there. Unfortunately my husband lost control of it, fell off, broke his collar bone, so I wouldn't recommend doing that. We are now in a dispute with this company over a $600 charge to our account and have still not received anything as to what this was for. Only damage to the one scooter was a mirror broke off, certainly not a cost of $600?? So, just beware.
Do you have any ideas as to where in GC you want to go? May just be easier to get a cab?? I'd also recommend doing the Sting Ray City snorkeling excursion, unless you've already done that, I'm just sorry we did'nt do that again.