On Monday, we visited the California Academy of Sciences. This was boy's absolute favorite adventure this trip. It's expensive so don't think about eating here on a family budget but do plan to spend the day to get the most bang for your buck.
Look for discounts and purchase your tickets online to avoid lines. Get there early, grab tickets for the planetarium, and visit the rain forest first. The aquarium was very busy the day we visited. I am not a fan of crowded dark tight quarters but the kids had a blast and that is what matters.