Mystic Aquarium in Mystic, CT. The New England Aquarium in Boston is also nice, but I don't know how far you intend on driving from home yet.
I haven't been since I was 12, but we had a WONDERFUL time when my family went - I'm the eldest of 4. We ranged in age from my then-4-year-old sister, 6-year-old and 8-year-old brothers and me (aged 12 at the time.) We also visited the Seaport, which I enjoyed, but I think my sister would rather have skipped.
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle