You should try to specifically visit it in Summer - the flowers along Michigan Avenue are incredibly gorgeous, not to mention the parks along Lakeshore, and the Lincoln Park - the botanical garden and the zoo. There is another, much bigger, botanical garden just at the outskirt of town, wellworth the time to go.
I know you like plants and flowers, you would enjoy Chicago very much in Spring, or in Summer. I have never seen a city town center has these many flower beds and boxes, big one too.
Field Museum is definitely worth to spend a day - their regular exhibits are excellent, though the special exhibits not always good.
Take a river/lake cruise or better yet, an Archetecture tour cruise, to admire the well-known Chicago highrises at sunset.
John Hancock building offers much better view than Sears and you can go to the restaurant floor for a very reasonably priced lunch - just a bit more than the admission fee to the observation floor, but get the same, or even better, view. (night time is much more expensive, but not necessarily better view, imo).
It is very rare for us to plan a trip in U.S. to include a particular city in the itinerary - Chicago is the only one.