Many of the O'Hare hotels are actually very close to one of the El stops - 5 minute or so walk, so depending on where you stay it may be easier to just leave the car at your hotel.

The Blue Line, however, doesn't drop you very near to most of the prime shopping/eating areas, and most trips involve either a change to another line - Jackson for the Red Line and then back north being one example - or a cab ride. State street is very close to Clark/Lake, but most tourists want North Michigan. I think it's just fine to walk up there along the river in all but the worst weather, but some people find it too far.

Parking downtown will obviously cost you a fair amount, but may just be easier. By being creative, some lots will give you a discount. As one example, if you park in the 900 North building and buy anything in the shopping mall the store will give you a discount on your parking fee. There are several areas within walking distance where you can troll for metered on street parking, but it's hit or miss and they usually have a two hour limit.