How about this:
- Friday night : arrive in San Fran, stay at the Monaco (?)
- Saturday: take in a Giants game (I'm a casual baseball fan, but am told it's the nicest park in the country). Maybe go over to the Ferry building (Slanted Door or Hog Island)
- Sunday morning: drive to Yountville
- Sunday/Monday/Tuesday: take in some wineries (Hess, Schafer, Phelps. Del Dotto, Arroyo, Anderson's Conn Valley, Charbay (for spirits) and others are on my list, but I'll keep it to 2-3...