If you have the money to dish out there's lots of amazing places to eat.
There's the original Uno's for deep dish, it's been a few years since I ate there but I remember it was good. I've had better but I've been spoiled by a place closer to me that has possibly the best (imo) deep dish pizza in the chicagoland area.
If you don't mind dropping ~$80 per person and like meat/seafoods there's texas de brazil. 70something item buffet, they bring meat out all you can eat that is to die for, lamb chops, steaks, chicken, etc. I've gone there 4 times and I've always loved it.
If you find one and have never had it get Garrett's popcorn.
Apparently there's some hotdog place that only opens after 4pm or something like that that serves any kind of hotdog. Literally any kind of meat i guess, I've only heard of it from word of mouth, though.
I would think this would be obvious but definitely check out and see if Navy Pier or Millennium park have anything going on, event wise. There can be some really awesome stuff. Millennium park also has some good views of some of the neat buildings.
Shedd Aquarium I've heard is pretty great, but I've never been. Museum of Science and Industry is a neat place to go to.
Also if you want to go onto the Willis(sears) tower but don't want to pay to get a view of the city you can go to the restaurant that's just a few floors from the top for free, you don't have to eat or anything but obviously you can't stay long.
There's also a lot of good breweries, I've heard. Check them out.
Obviously there's a lot more to do, but those are some of the things I enjoyed/know about. Walking around is great fun, especially with people. Tons of plazas and other cool stuff to see in certain areas.
