Museum of Science and Industry
link to another 2 for 1 coupon. Here's another 2 for 1, but it can only be used with an American Express. Free days for the museum can be found here. Hours are from 9:30-5:30.
This place came highly recommended, and I will not deny that it is amazing. However, we would have enjoyed it more without our children, or if they were a bit older. (My kids are 4&2.) It is geared more towards school-age children and up. Without small children I would recommend planning on staying for most of the day. For small kids 4 hours was more than enough. It was very crowded and my kids were easily shoved away from the interactive exhibits that they were interested in. (I hate reprimanding other people's children, but it had to be done a few times.) Also while we were there, several of the interactive exhibits were broken. My husband and I wished that our kids had the patience for us to read more about the exhibits. Mondays are the least crowded day at the Museum of S&I, if you have the luxury of picking that day. It has three large floors full of science exhibits that explain things like weather, physics, how the body works and technology. Even though it was crowded and we didn't get to see some of the things we wanted, I still felt like it was worth it and highly recommend it to anyone visiting the Chicago area. It's a fantastic way to teach your kids about science and technology. Cool things to check out there that we didn't have time for (meaning our children had lost patience): The Coal Mine Exhibit (the wait was an hour and a half) and Imax theaters.
This pic is great, but it doesn't do that place justice. Also when I was there I would say there was at least a thousand more people in the room.
This room was my kids favorite (also not a shot that I took.)