I was there in September. Following advice from other threads, we walked out to the National canopy. The agent was quite friendly and there was only one person in line (the bus discharged about ten people as we were finishing up so I'm glad we walked) We asked and received a convertible. The car itself was in sad shape (the top didn't go down, it lacked power and had a trunk that was full of sand). We should have taken the brand new HHR.
As a side note, we dropped the car off early in the morning when they were closed. They called 12 hours later asking for the license plate of the car we had rented. I'm guessing someone didn't check the car in correctly.
Enjoy your trip.