The Chicago Architecture Tour is a fan favorite. It is an hour long boat tour on the Chicago River highlighting prominent buildings and city history. Running ~$60/person, it is a great way to see and experience the city especially if it's your first time visiting!
A walk around Navy Pier is one of the best FREE activities in the city that gives you some of the most beautiful views of the skyline. On the pier you can find plenty to do for additional costs including the ferris wheel, restaurants, botanical gardens, art, museums, shops, and much more.