I did exactly what you are planning last year. Rented from Dollar at MCO, did a grocery stop and some other running around, then dropped the car at the Dollar counter at the Downtown Disney Courtyard by Marriott. If you plan to use their shuttle service, you'll want to call them and check on their hours, I think they are open 8:30am-4pm. You can return your car after hours (I did), you just use the drop box for your paperwork and keys, but then there is no shuttle. We just walked the few blocks to Downtown Disney (we were going to look around there anyway) and then used Disney transportation back to the resort (we took the boat back to POR, but we could have also taken the bus back). I did not find it to be a real hassle or very time consuming. I was with my 10 year old niece, so we both had to go, but if you have a couple of adults, just one of you could go and the other could stay back at the resort and use the pool or take a nap. We also went back on getaway day and rented a car for a one way trip back to MCO, I had no complaints about the counter service.