There are a number of car rental businesses in and around Santa Rosa. Several companies maintain counters at the Sonoma County Airport.
Enterprise Car Rental maintains a small office on Cleveland Ave (parallel and just west of the 101 freeway). Either exit College Ave or Exit Steele Lane to reach their location. Good for small to medium sized car rentals. They also operate another rental office on Santa Rosa Avenue (focusing on trucks but also a number of cars are in their inventory here).
Smith Rent a Car is a local car rental business located on Santa Rosa Ave. Often cheaper then some of the other more mainstream car rental companies. Have a larger group in town for some wine tasting and or touring? They do rent passenger size vans ranging from 8 to 15 seats. Friendly staff. Not always the cleanest vehicles. Very helpful and friendly staff.
Sixt Car Rental maintains two offices – one at the Sonoma County Airport and one in Fulton (a relatively short drive from the airport). Both locations offer 24/7 car return.