I'm looking for some good but not super crowded day hikes close to Chicago. Not Starved Rock, I've been there too many times and it gets too crowded. I really want to get out of the city and take a break out in nature.
I took a weekend trip to Washington Island (Door County, Wisconsin)-- you can take a hiker's ferry to the island and walk right up to a campsite reserved online. They have firewood and food for sale. The whole island is mostly a town, so if you bring bikes you can ride around! ...It's really cute and very much an escape from Chicago.
There are a lot of good ones with varying terrain. Castle Rock, the Des Plaines River trail, Indiana Dunes National lakeshore and Kettle Morrain in southern WI. All are within a couple hour drive of Chicago if you’re already ok with driving to starved rock.
The Des Plaines River Trail, North Branch Trail and Palos Trails are all great! You can hike or bike into the Chicago Botanic Gardens on the North Trail. If you enjoy bird watching, or want a looped trail, Orland Grasslands is also a good choice.
A lifelong resident of Chicagoland, I do most of my local dayhikes in the forest preserves southwest of Chicago -- the Red Gate Woods area, Cap Saur Holdings, Swallow Cliff, Waterfall Glen, and Black Partridge woods. I hike mostly off the beaten path on little known obscure trails where I rarely see anyone. I know where the **real** waterfall is near Waterfall Glen (not the man-made one everyone visits) and some rocky cliffs very few know about. I only go in the colder months, as I don't like heat & bugs and some of these routes can get badly overgrown in the summer.
If you don't mind a 5 hour drive, Shawnee National Forest in Southern Illinois has much to offer -- well worth a 3 day weekend or a full week. At the end of March I drove down early one morning for an overnight backback loop from the East Trigg trailhead, then drove back the next day. This loop included Cove Hollow, Jackson Falls, and Jackson Hollow. It was well worth the drive to me, but that barely scrathes the surface of great hiking & scenery in the southern tip of our state.
@KLK4210 , on the trip I described I backpacked in and camped near the base of Jackson Falls. I suspect you have in mind more of a drive in campground. There are many throughout Southern Illinois. Your best bet would be to look up the state parks.
A very good introduction would be Garden of the Gods. The short Observation Trail has spectactular rock formations, including the iconic Camel Rock. Kids love scrambling around on the rocks, but caution is required near the big drop-offs. The Indian Point Trail is longer, and another good option for kids. The campground is nice; I recommend sites further back on the right side as you pull in. Directly behind those campsites you can sit on rocks with nice views.
Other parks with great hiking and campgrounds are in that same general area. It's not difficult to find information online; the link below is a good place to start.