While he is in business meetings during the day, there are a bunch of things you can do! Like everyone said, shopping, museums, etc.
The Architecture Foundation does walking tours around the city, Chicago has some very interesting architecture, so I would definitely recommend those if you like that kind of stuff.
http://www.architecture.org/
The Historical Society also offers tours, but I believe most of them are on the weekends... so hopefully your SO won't have meetings then :)
The Wendella river boat tours are another great option.
Personally, I don't get the appeal of Navy Pier, and probably never will... maybe if I had little kids, I guess. However, there is a great Shakespeare Theater there, and there are evening cruises that take you out onto the lake for great views of the skyline at night. Also, on a Saturday night you would get to see the sky line and the fireworks which are every Saturday - or Wednesday (I think) - night from Navy Pier.
If the weather is nice enough, check before you head over here, you can always relax and get some sun on the beach too!
I don't want to repeat what others have said, you have gotten some good suggestions :) Oh, and another place for you to check for things going on before you arrive in Chicago is Metromix.com lots of listings of events, restaurants, etc.