The Museum of Science and Industry will be open this weekend during the NATO summit, and they're offering a special buy-one-get-one-free offer for all the visitors not going to the Adler, Art Institute, Shedd or Field - which will be closed all or part of the weekend.
With a printed voucher from the MSI website, you can receive one general adult admission free with the purchase of one paid adult general admission. The catch is that you have to buy your tickets in the building, not online. Special exhibits - like Mythbusters: The Explosive Exhibition, U-505 on-board tour, or Smart Home - are in addition to the general admission.
The two for one offer runs Saturday, May 19 through Monday, May 21, 9:30 a.m. to 4 p.m. each day.
MSI in Hyde Park is far enough south that there are no security restrictions from the international gathering at McCormick Place. But it might be a good idea to check traffic and plan alternate routes if you're coming from the north. You can find constantly updated info about road closures at notifychicago.org or chicagonato.org. MSI will also have some recommended routes to make getting there easier.
For more on the musuem, check out MSIChicago.org.