List with Links:
Chicago History Museum - Tours and events by type offered regularly around town.
Bobby's Bike Hike - Guided bike tours for all levels.
Chicago Trolly & Double Decker Bus Tour - Hop on/ hop off. Great way to quickly cover ground.
Chicago Architecture Foundation River Cruise - The standard river cruise that locals know is a must for the best views of downtown.
Also, visit Explore Chicago (official) for a searchable list of tours.