There is one privately-owned Avanti that flies into San Jose Mineta Airport on a semi-regular basis (once in a few days). Or you could go to Reid-Hillview and watch all the V-tail Bonanzas doing flight training. In fact, generally small airports with small planes are more interesting -- there's greater variety among the smaller planes, for understandable reasons (lower development cost, for instance). Military fields anywhere are a good bet, what with all the fighter jets and whatnot flying around (Moffett Field used to be a spotter's paradise until it was decommissioned a few years ago). For the REALLY hardcore spotter who wants to see some REALLY unusual experimental flying saucers, Groom Lake (aka Area 51) might be a good spot -- that is, IF you're willing to risk getting yourself arrested for spying. Edwards AFB outside of Palm Springs could also be a good spot on a good day, but it's a long drive from town and not a very active base on most days.