We are lucky to live about an hour and a half from Chicago because that is our go-to place for any kind of vacation. It can easily be a one day trip.
We bought a membership last year for the Shedd Aquarium (only $10 more for our family to buy a membership vs. a one-time visit!). We went to our favorite exhibits and then went to the Museum of Science and Industry. Now we have a membership there for the next year!
As part of the membership, we got to tour the U505 which was amazing. The kids really enjoyed it too and we talked a lot about it on the way home.