The loop encompassing Pleasanton, Milpitas, Mt Hamilton, Mines Rd, Livermore is great fun. Mines Rd has never been too busy when I have been down it. It really is cracking fun and perfect for the FRS/BRZ as power isn't needed here.
I also went out towards Patterson at the 1/2 way point of the loop one time and then took I5 and 580 back around. It is busies because of the campgrounds and also has some 'free range' cattle to deal with.