I rented a Sebring convertible form Avis in Vegas and then "Did" California similarly to your description. While they did not guarantee a car make or model they did guarantee a convertible (better that other car rental places I’ve used). I called the Vegas office and they said all they had where Sebring's.

Convertibles in CA are the way to go. However I also rented a Grand Am GT and carved up some canyon roads. That was also cool.

Do the “Avis Preferred” thing. They take you right to your car and you don’t have to wait in those god-awful lines.