Jackie and I took a trip to the Monterey Bay Aquarium some time ago. It's a pretty amazing place, comprehensive, informative, and entertaining. It's worthwhile whether you go with 987452903475 kids or not. It always seems to be fairly crowded, but the place is huge, and most visitors are good about viewing a tank for a moment, then moving on. There are gigantic tanks, tiny tanks, interactive exhibits, a fish petting zoo, and, since the building used to house a cannery, a cannery exhibit.
The jellyfish are particularly eerie and mesmerizing. Turns out they're poisonous!
Of all the seahorses, these are the strangest and most beautiful. But the rest of the seahorses are nothing to sneeze at, either.
The jellyfish do spend most of their time "upside-down".
Sea otters feeding time, is when they're most active, but also when the biggest crowd shows up. Check the feeding schedule, and find a spot about ten minutes before the feeding starts.
Be sure to make time for a visit, downstairs, to the Giant Octopus, another big favorite of mine. He lives pretty much in the dark, so photography's out.
Oh, and although the Aquarium is located in an area that is the bastard step-child of Pier 39 (rock candy, fridge magnets, etc), I did manage to spot this sweet ride there.
It's a hurried half day trip from SF, since it's about a two hour drive, going the fastest way. However, you should take the two-and-a-half hour drive along the coast. This scenic route features plenty of views, twisty roads, and historic WPA projects along the way.