Cabrillo Marine Aquarium. It's located in San Pedro, CA and the first time we visited it was a bit of a fluke. I was looking for things to do before boarding our cruise ship, and I found this aquarium that was free (donations are suggested, but not required. We always donate what we can.)
It is a wonderful place that has a variety of exhibits, including a touch tank. In the touch tank, you can touch sea stars, sea urchins, sea cucumbers and anemones. You can also see, (but not touch!) a couple eels and some bamboo sharks. There is a research component to this aquarium, and you are free to visit their research area which includes tanks where they breed sea animals -- including jellyfish (did you know jellyfish are plankton?) and sea horses.
They also have an area set up with microscopes and a variety of things to view. This picture is of Nate checking out a sea urchin under the microscope.
It has been fun watching the boys grow up visiting this aquarium, and it is interesting to see the different things they are drawn to during each visit as they mature. The area with microscopes was especially interesting to them this trip, as well as the exhibits about whales.
If you are ever in the area, I highly recommend a visit to Cabrillo Marine Aquarium. I know you won't be disappointed.