We are Emerald Club members at National and we just pick any car we want from the Emerald Aisle, drive up to the checkout booth, and go. All my credit card info is stored with my profile, so I don't have to give them anything to checkout but my ID and Emerald card.
Several other agencies have similar programs, but some of them still require you to wait in line (it's supposed to be a faster line, but when I used Dollar's, it took just as long).
There are several ways to get a free Emerald Club membership... I don't know of any off the top of my head, but I'm sure someone else here does.... (maybe check the transportation board??).
Good luck!