The New England Aquarium is built around a large central salt water tank. The exhibits spiral up around it. At the bottom is a popular penguin exhibit. You can shine a fish shaped light on the floor of the tank and the penguins will follow it. At the top of the tank you can watch them interact with sharks. I remember the electric eel being popular to watch (complete with a voltmeter nearby) and a tank where you can interact with small water animals (starfish, etc). The entrance cost isn't too bad compared with other aquariums I have been to. There is a good sized, yet pricey food court attached for lunch.
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