You're much more likely to see fascinating animal behavior if you hang out for a bit at an exhibit, preferably one featuring active creatures. Don't try to drink it all in. The ocean of information at an aquarium could overwhelm your child. If possible, consider several short trips. Otherwise, try to break up the day with snacks, lunch, and a romp on the playground, if one is available.
Some aquariums provide models of sea animals for children to play on as a nice change of pace.
For example, sea horses change color to blend in with their surroundings and sit very still. And sea dragons have evolved frilly, lacy fins that make them look like pieces of drifting seaweed. Picking up a sea star or sea cucumber or touching the back of a crab is a thrill for young kids, who are eager to check things out with their sense of touch. The more senses your child uses during the visit, the more enriching and fun the experience. Suggest that your child show the same courtesy to plants and animals as he shows to friends at daycare or preschool.