The Natural Science Center has been one of my favorite places since my own childhood, and is even better now that I have three kids of my own.
Membership for city residents, at under $50/year, is a true bargain, and well worth it. I love being able to stop by on the way home from school for an hour or so, or to be able to head over to the Center on a rainy day.
Some of the exhibits are still the same as they were when I was a kid, but the Center is in the process of a huge expansion and upgrade. The new exhibits are all top-notch, and even the old ones are enjoyable despite showing their age.
The basement of the center, with the sea-life touch tank and the reptile/amphibian petting room is my family's perennial favorite. So are the lemurs!
We nearly always end our visit with a stop at the interactive play area, where the kids can play in a pretend grocery store, make up a puppet show, or do a dozen others things.