You can ride muni with unlimited times. Also, if you have fast pass, present your fast pass to the driver and you can take cable car free. If you live outside SF City, this is the transportation that connects you to San Francisco. BART is very punctual and fast. Of course, the fare rate is relatively higher.
Cable car is historic and it is fun to ride.
You can go to China Town, Fisherman's Wharf, Ghiradelli Square, and Lombard Street with Cable Car. Not very familiar with this system, but if you are at the airport and don't feel like to get a taxi, samTrains can bring you to San Francisco.