If you rented on-line and aren't using a coupon, you can check-in on-line. There would be a place on the confirmation email that check-in. We did this last year and you print out all the info and then just go to the garage and show the paperwork and license at the exit. If you are using a coupon there would be a place on the confirmation for you to enter most of your information ahead of time and then you would just go to the kiosk near the
Alamo desk in the airport, finish the info and then go to the garage.