I like visiting museums. I am fascinated by engineering and basically how stuff is made. Recently, while in Chicago, we toured the Museum of Science and Industry. Like everything in Chicago, it was not a cheap visit. 
With so many things to do and see, here are my favorites; interactive exhibits, like the tornado and hot-air balloons from the Science Storms, an actual 727 airplane suspended from the building, vintage firetrucks, and displays of vintage & modern aircraft and spacecraft!
Above the basic admission, they offer extra tours, exhibits, and other “add-ons”.
A captured German U-boat (submarine) basic exhibit is included in the general admission, with option to add-on a guided tour. If you have time, definitely add the tour! Great for children 10+, the tour is informative and just really neat! Two doorways are cut into the side and you walk (almost) the entire length of the boat! It was amazing to see the navigation stations, kitchen, bunk beds, torpedo storage, and so much more!
Our final add-on was the coal mine tour. This is the original and has been a permanent exhibit since 1933. The coal mine is a great family activity, perfect for Children 5 and up. The tour starts in a working coal mine elevator and includes a quick train ride and lots of fun-facts. 
There is working equipment that spans old and modern mining techniques.
There were many other exhibits and activities that we wanted to see, but our trip was cut short due to lousy transportation. If you are visiting from a Chicago-suburb, it is much easier to drive directly to the museum and pay to park. Otherwise, you may experience a significant delay on the bus or train. There was an event downtown that resulted in many bus routes being detoured, so we ended up in the worst part of South Chicago for a bus transfer. I would not even recommend walking from the train station at 55th/56th. If you really want to take the bus, or have no other options, the X28 is the best option.
I really enjoyed our visit to the museum and will plan to go back on our next visit to the Chicago area.
June 2014 Update – Made it back to the museum and brought my two favorite little girls, ages 4 & 7 years. They loved the coal mine tour, and earned a ton of stickers for participation and correct answers during the tour.
We went on a “free” day and the museum was packed with people, mostly school children. The extra cost add-on tours were a break from the bus loads of kids and other families enjoying the museum at no cost. For a family of 4 or more, the free day is really worth it, if you can handle the crowds and noise.
There is currently an add-on tour of Treasure of the Walt Disney Archives that displays various Disney props and costumes. There is also a hands-on drawing portion of the exhibit. The girls were mildly interested, but spent most of the entire 10 minutes in the exhibit starting at the cartoon clips shown on multiple screens. I adore all things Disney, but this was just a waste of time & money for the girls and adults. Save your money and spend 10 minutes looking over the images from the website, they offer better views than those in person behind the display glass.
The Fairy Castle was a nice break from the crowds, even thought it is part of the free collection the exhibit enclosure automatically limits the number of people at a time. The multi-level castle has an enclosed garden, ballroom, library, and so much more.
There are many interactive activities and games for the kids. Including solar car racing, life-size hamster wheel, hot-air balloon launches, trains big & small, and a miniature rocket launch.
This is absolutely one of my favorite museums, and once again, one of the highlights of every visit to Chicago!