The parking downtown will be a hefty charge. I'd suggest cabbing from ORD to your hotel and back and then renting a car downtown for the run out to Northlake and back.
Another twist to throw into the mix. If for some reason you WANT the car overnight (parking will be over $30 in that area, I expect), you can rent by the hotel and return it to ORD without extra charge from most of the major car rental companies.