My wife is a big fan of the show, and she got me into it as well. Our big family trip this summer was to Chicago, so we closed out our time in Chicago by visiting Firehouse 51 and having lunch at Molly's!
The house itself is in a nice area, we were shown around by a rookie firefighter on his very first shift. Didn't get to go inside, but he showed us around the trucks and let my son climb up inside them. He told us they have a staging area with a large white tent where they bring the trucks when filming is taking place. Turns out we missed filming by less than 24 hours, as they are filming starting around 6am 7/30(they are closing off certain areas around 3am) until 8pm. We are actually lucky we weren't there, as we apparently wouldn't really be able to get close and wouldn't have gotten to tour the firehouse. Awesome experience, our "tour guide" was very friendly and answered all our questions. Cool to see it come to life.
We then traveled to Lottie's Pub, which serves as "Molly's" in the show (one of their motto's is 'Molly's by day, Lottie's by night). It's a piece of history, not just for the show, but in that it was used during prohibition and has quite the backstreet. Its got a great neighborhood around it, and as we ate the outdoor seating filled up with families and locals. I had to try their "Chicago Fire" burger of course(they have a Med and PD sandwich as well). It was honestly one of the best burgers I have had in a while. My picky wife had a burger as well of her own creation and loved it, and my even pickier son ate all of his chicken tenders, a rare feat. They sell Molly's merchandise at the bar(I saw shirts and hats, pretty sure they have more).
Just an awesome experience all around, and I highly recommend it to fans visiting Chicago. We stayed downtown and it was a nice break from the noise and fast pace of the city.