>>311362I take it North Suburbs? South Suburbs is packed with forest preserves and corn fields. There were lots of nights in high school where we packed up a truck and a keg and hung out in a corn field drinking and lighting off fireworks; cleaned up everything morning after and never had any problems. Never thought about camping in a forest preserve, but that's because cops ticket like no other when you're parked overnight in the suburbs. I'd suggest Wisconsin and the Ice Age Trail for some backpacking fun. Or Starved Rock if you can find a spot to stash your car.
Also as per OP, I'd recommend a bivy bag for stealth camping. Less to set up and military surplus bivys will help with the concealment better than a big orange or blue bivy. Cheaper too.
BUT, as suggested previously, there's not much you'll really need with a low of 50 degrees for one night. A nice fire and some whiskey. Maybe a $5 tarp as a lean to if it rains. Some of my best camping trips have been with very little. You can even get by with a fold up chair, blanket, and a fire.
