1. Chicago Architecture Foundation River Cruise
This was probably one of my favorite things to do while I was in Chicago. The boat was fairly easy to access as they put out a ramp to wheel right on. You have a clear view of the city as you go along the river and there is a tour guide which is easy to hear. If you're an architecture nerd or just want some interesting views of the city, it's really worth the time to make this trip. Just make sure to do this on a nice day as the boat I was on didn't have any cover.
2. Art Institute of Chicago
Ranked #1 on Tripadvisor for a reason. The Art Institute is not only a really interesting building in its own right but houses some amazing, famous works of art. If you ever wanted to see Grant Wood's American Gothic in person then this is the place to go. The Art Institute is within easy walking distance of Millennium park so its great to do both in the same day. My suggestion is to have your lunch in the park to break up your day.
3. Millennium Park and the Crown Fountains
This is a huge park that goes on for blocks near the Chicago waterfront. You could spend a long time just wandering through it and enjoying the day if you're not in a rush to be any place else. The Crown Fountains are a feature in Millennium Park that I personally really enjoyed. Opened in 2004, the fountain is composed of a black granite reflecting pool and two 50 foot tall video screen towers. The video screens show people's faces and water shoots out of their mouths from time to time. It's a great place to just hang out and people watch on a nice day.
The other big feature of Millennium park is Cloud Gate aka The Bean. The huge polished metal sculpture reflects and distorts the Chicago skyline. You can walk right under it and look up to see yourself reflected, which is pretty cool.
4. Shedd Aquarium
One of the largest and most popular aquariums in the US. The Oceanarium exhibit has one of the largest indoor tanks at 2 million gallons. This tank hosts large marine mammals like sea otters, sea lions and beluga whales. There are also multiple tropical and coral reef tanks each with their own theme. Tickets aren't super cheap but you can get in as part of the Chicago CityPASS. Regardless, I think there is enough to see and do here to make it worth going.
5. Michigan Avenue
The Magnificent Mile section of Michigan Avenue just to the north of the Chicago river is one of Chicago's prime shopping districts. Mostly a tourist area with lots of high end shopping and restaurants. It's still worth the walk, even if just to window shop. Near the north end of the strip is the Historic Water tower, one of the few buildings to survive the great fire. Another interesting building to see near the water tower is the luxury Drake hotel, one of the most famous and oldest hotels in Chicago. On the south end of the street near the river is the John Hancock Center, which has a great view from the observation deck. South of the river, Michigan Avenue goes past a few of my other picks including Millennium Park and the Chicago Institute of Art.