Anything near Navy Pier will be expensive and possibly have a parking fee of up to $50 per night. Every large chain is represented nearby, and most should be okay.
I would check some discount travel websites, sometimes I see deals at Kimpton or Swissotel show up on Travelzoo.
If you want opinions on a specific hotel let me know.
You did not ask, but an awesome and inexpensive for downtown resturant to eat is called Yolk. Check out their menu online, it is as good as it sounds.
The closest hotel near Navy Pier to accomodate 6 people would be the Embassy Suites Chicago Downtown/Lakefront located at 511 N. Columbus. There is another Embassy Suites further west at 600 N. State Street that tends to be a little cheaper. Both locations include a full breakfast, free evening snacks/cocktails and have an indoor pool. I've always received best rates by booking directly through their website and using my AAA membership.
More budget oriented hotels near Navy Pier would be the following:
Hilton Garden Inn at 10 E. Grand
Hampton Inn and Suites at 33 W. Illinois
Good luck with your search. Hotels in Chicago tend to be very expensive, especially during certain times of the year (ie. summer).
Are you visiting NP for one day and wanting to spend the night in Chicago?
Agree with previous posters, Hotels will be very expensive in the city. I would drive in early, spend the day and drive out and stay elsewhere. Or just pay the $$ if it's special day.
Sorry, I haven't stayed at any downtown hotels for that very reason.