Chicago is a wonderful city with a good mass transit system. Your post did not mention your age or your interests, so I will keep this general.
The Sears Tower has a great observatory but tickets run approximately $16 per person. A better way to get almost the same view, in my opinion, is to visit the Signature Room on the 95th floor of the Hancock Building. The bar is on the 96th floor and the view is fabulous from both floors. The drinks are moderately priced for Chicago but, if you go for lunch as I suggest, you can get the view and a wonderful all-you-can-eat lunch buffet for $18 p/p. The complimentary flat breads are wonderful and the chef does a wonderful job with presentation of the buffet dishes. More bang for your buck and you can enjoy the view as long as you wish without a lot of children running around. If you opt for the Sears Tower, be aware that this time of year lots of school groups take field trips to the tower.
You did not mention if you had a place to stay yet but I highly recommend the Hilton across from Buckingham Fountain. Some of the rooms have lake and fountain views, the rates are good and the service is second to none. The decor is stately and reminds one of entering an era gone by. They have an on site bar which is usually packed and two or three restaurants and a salon downstairs.
The Magnificent Mile, of course, has tons of window shopping opportunities. There's Water Tower if your wife wants to do some shopping. You can also visit Millennium Park with the infamous "jelly bean" and enjoy a nice stroll.
Better take a trip to Wrigley Field before they decide to renovate/tear it down when the Cubs are sold. Check out the website for tour times and reservations. The Cubbie Bear is right across the street where you can slam down a few cold ones or watch the game if you can't get tickets.
The Ferris Wheel is up and running at Navy Pier and is a romantic place to snitch a kiss with your sweetie. But the Stained Glass Museum in the basement is fabulous - and free. There is an eclectic assortment of shops and restaurants to try too. BeeBop Cafe serves up tasty BBQ and some great jazz. Bubba Gump Shrimp Co is here as are Riva and Charlie's Ale House.
You can check out their entertainment schedule at http://www.navypier.com/home.html.
If you are into the bar scene and want to be "seen," you can visit Martini Park (but be prepared to wait a long time and drop a bundle), Rockit Bar and Grill has been known to host some of the Bulls players on their off time and Gibson's is a favorite with Bears players. House of Blues is famous and comes with a price tag to show it. If you are into live music, grab a City Scene or the Red Eye and check out what might be happening in the smaller clubs to save some $$ but still have a good time.
If you are looking for a moderately priced and historic restaurant, try 17 West which is located in the old Berghoff building. The bar is great with a variety of beers on tap and good pricing. The restaurant offers a nice selection of well-prepared entrees in the $20-30 range. Service is excellent and the building is truly a landmark.
For a significantly less expensive and totally crass and down to earth insulting good time, try Ed Debevics. The wait staff will serve up your burger and shakes with a whopping portion of dry humor and smart-mouthed comments. They pride themselves on being the rudest and worst dancing staff in all of Chicago. You can have a good laugh and a good burger/shake here for under $35 a couple.
On the free-to-visit-list would be Union Station where you can walk down the same stairs Kevin Costner did in the "Untouchables," have some nuts at "Nuts on Clark" and check out the daily commuters coming and going to suburbia.
If you are looking for something specific, just drop me a line and let me know. I am leaving for Amsterdam on Friday but I'll get to it before then.