There’s a Hostelling International (HI) hostel across the street. It has some private (two-person) rooms ($119+/night) and some multi-bed dorm rooms ($34/night).
There are also lots of Airbnbs in Chicago, Oak Park and Forest Park; the latter two are a short commute via public transit.
Attendees do not need to rent cars. There’s excellent public transit to and from both airports, and getting around the city is easy by train, bus, cab and Lyft. Parking is expensive!