I agree with the idea of picking up the car, and using it as the luggage holding place while parked in the garage (either 5th and Mission or Sutter-Stockton garages - the latter is closer to Chinatown). From either garage, a cable car stop is only one longish block away.

Another advantage to the car over BART is the ability to get to a couple of places outside of the Central downtown area. Specifically, Golden Gate Park (don't niss the Herzog and de Meuron-designed DeYoung Museum, even if you don't have time for an extensive visit); and, time permitting, a quick drive across the Golden Gate Bridge (stop at the view point to see the SF skyline, then head back - return to the freeway is very easy, although you could also consider lunch in Sausalito).

http://www.mccullagh.org/db9/1ds-4/de-young-museum.jpg