While folks will mountain bike in a number of areas in the preserves, the two official sites are the Palos Forest Preserves in the southwest portion of the county and Deer Grove in the northwest. Our friends at CAMBr have the scoop on mountain biking in the preserves, and volunteer workdays for maintaining and building trails. They do a great job with educating users about staying on official trails and advocating for trail users.
If you want terrain, good trails, and beautiful nature, check out either of these two spots. Palos has many more trails that offer many different lengths and skill levels. While Deer Grove is a bit more straight forward. Either way, these are some of the best trails south of the Wisconsin border.
Mountain biking off trail does tremendous damage to these fragile ecosystems, never mind the fact that it is totally illegal and can get you up to a $500 fine. So, please, check out the maps and plan a day where folks have worked hard to give you a great experience.