I like Alamo. If you book on line, you can do a check in on line. All you have to do is go to the size / class of car you rented, pick your car and drive off to the gate. Make sure you have your paperwork with you, though. I don't want you to get arrested for stolen vehicle. ^_~
Even when you return your car, just drive up to the drop off area for Alamo on the second floor and be on your merry way. If there's no attendant there, be sure to record your mileage so you can let the staff at the counter know.
Have a great vacation!