If you want "nice" then go to National . About 15 years ago while vacationing in California many years ago my wife had tossed her wedding ring up onto the dash not thinking that it could fall down into the defroster vents. Well it did. I took the car to a local Chrysler dealer but he said it would be very complicated and costly (about a thousand $) as they would have to remove the whole dash in order to get at the duct work..!!
The wife and I said ah forget it and at the end of the vacation we returned the car to National at the airport (SFO). While they were doing the paper work I casually mentioned that if any proceeding customer of that car should happen to find a wedding ring that it was ours. The woman at the desk was shocked to hear the story, she got on the phone, called the rental return crew and told them to pull the car from service. She took my home address and said she'd send it back to me if they found it. I thought yea right, whatever..
3 weeks later there it was in the mail....!!!! They didn't even charge me for retrieval of the ring..!! It goes without saying I have never used a different rental agency since.
We always use http://www.hotwire.com/car/index.jsp whenever renting a car in the States. Hotwire offers cars from the major rental companies (Hertz, National, Avis, Budget, Alamo, etc.) but at significantly lower prices. You won't, however, know which company you're renting from until you confirm and prepay your reservation. Hotwire rates do not include insurance, but neither do the rates listed by the major car rental companies.
You can use http://www.kayak.com/cars to compare the rates offered at different car rental sites, including hotwire, priceline, and expedia.
I don't know if you get a better deal by paying for the upgrade when you pick up the car. At Hotwire there isn't a large price differential between an economy car rented at LAX (Los Angeles) for $20.95 per day and a full size at $23.95 per day. A premium car is $33.95. I've gotten lucky quite a few times by reserving a lower level car and then getting upgraded a few levels because they were out of the size that I reserved. If I know the market where I'm renting a car, I've even used this technique knowing that the chances would be in my favor that they would have to upgrade me.