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.