Rent a car IN the city at a rental company location, rather than paying all the fees at the airport rental sites. You can take public transportation, including the subway, from either San Francisco or Oakland airport into San Francisco and then rent a car when you need. It. Very expensive to park a car on the street or overnight in a garage thus use for day trips. almost 7 years ago
Your question is impossible to answer effectively without more information.
How are you getting to SF--from an airport, which one?
Where are you staying? (Parking can be very expensive, depending on where you're staying.) How long will you be there?
What do you want to do there?
Where do you want to go and what do you want to see?
Will you be going outside of the city?
There is no "subway" in SF. There is BART and Muni.
The more information you can provide, the better your answer will be.
Have fun in one of the greatest cities in the country! almost 7 years ago