I never miss the Monterey Bay Aquarium. It is simply awesome and unlike any aquarium you'd ever see...I mean, it's got to be the best in the world, or at least in the top 3. They have a deep ocean tank that's got huge sunfish and sharks in it...they even kept a Great White (juvenile) in it for a bit (released now). It's quite a drive down to Monterey from 'Frisco (about 50 or 60 miles), but I always, always go. I usually grab a bite to eat near the aquarium where you can watch Sea Lions bask from your dinner table (but be prepared for lackluster tourist-trap food), then hit the aquarium. If I leave early enough, I sometimes hit the Santa Cruz boardwalk on my way back to San Francisco, mostly for the nostalgia of my parents taking me as a kid (or because the "Lost Boys" from the 80's film hung out there..in the movie anyway), but it can get kind of scary there lately (population wise).
The aquarium is great at feeding time (usually around 1 pm if memory serves). The kelp forest is awesome with two-three story tall tanks (also the sea otters). The Monterey area is beautiful to begin with anyway, so it's nice to visit anyway, especially if you like Steinbeck.