It depends on what you enjoy doing but here is my short bucket list for friends who visit:
Chicago Blues and soul food at Kingston Mines on Halsted
The Art Institute (check their website for the free evenings)
Millennium Park
Chicago movies in the park (Google and check the schedule - free and bring a blanket early)
Also, if you like afternoon tea, I suggest the Drake. It's fun with a group but a little more $$$.
Please do enjoy the city!