Lick Observatory is one of the Bay Area's great science landmarks, perched on Mount Hamilton above San Jose. The drive is slow and winding, but the reward is a combination of historic telescope domes, astronomical history, and huge views across the Diablo Range and Santa Clara Valley.
It is worth treating as a half-day trip rather than a quick detour. Check visitor access before starting up the mountain, bring layers even when the valley is warm, and do not underestimate the road if you are prone to motion sickness.