As a Chicago born expatriate (I live in the suburbs) my suggestion is to stay away from anything in the downtown/lakefront area. 20 minutes from downtown is more like 30-45 minutes due to traffic congestion and road work. (I stopped calling it construction. More like pushing dirt around.) Parking downtown will cost you $40 -$80 for your time. Sites mentioned are not close to each other so you'll be moving your car multiple times, with the attendant Parking fees.
Do not ride the "L". As an out of towner you will be a target for roving gangs of thieves. Unless the family you're visiting is a police officer who will accompany you, I'd suggest a different venue. (Just last week a security guard for one of our prominent television stations was killed in a carjacking attack.)
Depending on where you are staying, I can offer a few suggestions.
South suburbs, go to Flossmoor Station, an excellent brewpub located in an ex-IC passenger station. The beers are great and varied in in flavor profile. They're right on the main line so you can watch trains pass.
Southwest suburbs, come to the Chicagoland lionel railroad club's layout and open house. We're located in New Lenox Illinois, along Interstate 80. Check out our website, CLRCTRAINS. COM. WE'D love to have you.
Western suburbs, if you like take the Metra BNSF service to Aurora Illinois (the end of the line) and go to 3 Brothers roundhouse. Another brewpub located inside the CB&Q's 1860's era 360 degree roundhouse. Another great place, once owned by the late great Walter Payton. You can also drive there via Interstate 88, hope you have an easy pass or similar because we have toll roads here.
North or northwest, if you're staying near O'Hare Airport then got to either des plaines hobbies or Chicagoland hobbies. They're both close to each other and worth the visit. There is an HO and N gauge model railroad club in Elmhurst Illinois but I don't know much about them.
Near west suburbs, Berwyn hobbies is a good choice.
FYI, that weekend is St. Patrick's day and many restaurants will be crowded with long wait times. Best to make a reservation for dinner, lunch maybe not.
The weather in Chicago is extremely variable in March, expect anything from 20's and snow to 70 and sunshine.- sometimes in the same day!
I don't mean to burst your bubble but Chicago is not the same place you visited 30 years ago. I used to go downtown several times a year via train, now only once a year to see my financial advisor and then head home right after.