I recently returned home to San Diego after living in Chicago for four years. What a great city!
If you go, you must go to the Park Grill at Millennium Park. Their food is seasonal and absolutely fantastic! Visit the John Hancock Building's Signature Lounge at sunset ( get there early for a good seat )and treat yourself to a cocktail. You'll never see anything like it and you don't have to compete with kids visiting the Observatory for a great view. Hungry? Make it an evening and stay for dinner! The food is pricey but, a must do Chicago event.
Out shopping and need a quick snack / cheap eat? Wow Bow at the Water Tower Place has super yummy bow buns and pad thai. Don't forget the cranberry ginger soda!
Going to the North Side? Run to Tango Sur, you don't have to fly to Argentina for authentic gaucho food.
The Gold Coast neighborhood of downtown has good Italian food but, in the heart of downtown Volare is the best I've ever had! I use to work at the front desk of the Ritz Carlton Chicago and recommended Volare to all my hotel guests and never received a complaint!
If your time is limited in Chicago a few must do's are; visiting The Field Museum, The Shedd Aquarium, and The Adler Planetarium. Chicago is very proud of it's Architecture. There's a story behind every building and every bridge. Take the Architecture Boat Tour ( the two hr tour if you can spare the time ), it's so interesting!
Before you go, you must read The Devil and the White City, by Eric Larsen. I don't want to give the story away but, know it part wacky, weird, and all true! The book also explains how Chicago became the Second City and it has NOTHING to do with being second to New York.
Enjoy your trip to Chicago from someone who lived there and loved it ( minus the snow )!