Decatur is not really a tourist area. The people who live here, either make their livings in manufacturing or farming, or by offering services to the other people, who are manufacturers/laborers/farmers. As such, this is not a tourist-heavy area. The one good benefit of being in Decatur, is that it has relatively close promimity to areas which do have touristy stuff. We are 3 hours from Chicago, 2.5 hours from St. Louis, and only 45 minutes from either Bloomington-Normal, Springfield, or Champaign-Urbana.
Springfield has some amazing things to do on a 1-day trip. The Abraham Lincoln Presidential Library and Museum is an awesome, world-class experience, and there are associated Lincoln sites, which are not to be missed. Bloomington-Normal is home to Illinois State University, and Urbana-Champaingn is home to the University of Illinois, so these areas have your typical university town fare, including plenty of nightlife and good restaurants.
If, however, you are just going to stay in Decatur, there are some things to do. If you enjoy golfing, we have some decent golf courses, and there are some nice parks, such as Rock Springs Nature Center, which offer fun outdoor activities. Merchant Street Downtown offers some unique boutique type shopping, and Forsyth, an area North of town, has a mall and chain restaurants. For finer dining, I'd recommend Tuscany, and Italian restaurant/steak house, which is located near Millikin University campus, or the Beach House, which is located on Lake Decatur. The Beach House is truly excellent dining, and eating there will make you feel like you are NOT in Decatur, which is a plus!
If you have kids, Scovill Zoo and playground is fun, as is the Children's Museum of Illinois, located just next door. Rock Springs also offers lots of children's activities.
The first weekend in August is the Decatur Celebration, which is Illinois's largest street festival. It's a pretty big thing to do here, and offers vendors, live music, and rides.