Even if they don't have a concierge you can still rent a car for just a day. There are quite a few car rental agencies in town. What I would suggest though is, if you know which day you want the car, make the reservation ahead of time. You can use Costco, and also AutoSlash I think, to book a car in San Francisco. We did this for a trip several years ago. I booked the car for around $35 for the day. By the time we arrived a one day rental was going for around $100. Turns out we were there during the World Series and a couple of the games were being played in San Francisco. You can never tell what will be going on.
There is an Avis car rental about a one minute walk and Alamo/National car rental about 5 minute walk.