Search the Hertz Fun collection........they usually have a nice selection of convertibles you can choose from.
If you want a more mundane car or possible a SUV, Alamo usually works out well. For our trip to San Francisco, I booked a compact knowing we'd get bumped up at no charge. Lo and behold, at the airport all the compacts were taken out. They actually had no supervision at all, and the keys were just stuck in the keyholes of the available cars and you drove out on whatever you felt like (the attendant at the gate never cared). So we got a free double upgrade and paid a compact rate
. Be careful though.............SUV means more fuel consumption. Gas is cheap in the US, but it's not free. Think about that before driving off in some Chevy Equinox or Jeep Cherokee.