At first glance, it may not seem like there is much outdoor recreation around C-U, but if you have transportation there are several great parks and hiking spots within an hour's drive from town. If you're willing to venture farther afield, even more opportunities are available.
Kickapoo State Park - Historically mined, this area includes terrain that's hillier than usually found in central IL as well as several lakes and wetlands. The park has a 7 mile loop as well as some shorter trails that are good for running and walking as well as mountain biking. Bring a map, as they are not available at the park. You can print one on the park's homepage.
Near Kickapoo, Middle Fork State Fish and Wildlife Area also has a substantial number of trails and similarly varied terrain. A nesting pair of bald eagles can sometimes be spotted.
Champaign County Forest Preserves - The five forest preserves offer varied opportunities for outdoor recreation, including hiking, fishing, boating, and wildlife viewing. Follow the link to see details.
Allerton Park and Retreat Center - Near Monticello, IL, about a 45 minute drive from C-U, Allerton is a conference center, retreat venue, and wedding spot along the Sangamon river. It also boasts about 8 miles of natural surface trails and several varied formal gardens. If you haven't been there, you must go at least once.
State Parks: The Illinois DNR has detailed information on other parks, organied by region.