It is mostly just the headquarters, but they do offer tours and have a gift shop. Before heading over, check on their website (nasa.gov) to see if the shutdown has effected visitor operations though and check on tour times.
There's only a few display items, and it's not really set up for younger audiences, but they have a pretty nice giftshop!
The west lobby area has an Apollo era spacesuit, moon rock and a couple other items right next to the gift shop.
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