There are plenty of fun things to do here, you just need to know where to look. For historical sites, Lincoln's new Salem is just outside of Springfield, there's Kaskaskia, plus there's Carthage, which has the jail house where Joseph Smith was shot and Nauvoo where the Mormons lived before going to Utah.
There's also many, many historical sites on Chicago and Springfield.
For wilderness there's the Shawnee National Forest in Southern IL, Pere Marquette is another good place with much uphill hiking. Another place that falls under both this and the history category would be Starved Rock, which like Pere Marquette has a campground and uphill hiking. There are also many more hiking/biking places along the Illinois and Mississippi River.
Out in the country you will be surrounded by cornfields and more cornfields.
Most of the things going on in Dekalb are people at bars, Rosy's Roadhouse kinda blew up as an awesome bar but I don't know from personal experience. House parties by frats and so on.
Time to time NIU convocation center brings awesome artists, comedians or talents. Egyptian theater sometimes has things to go visit and do. Hopkins park has a small trail you can run, bike and so on. I believe it extends all the way into the NIU campus by the soccer fields by Barsema it follows the Kishwaukee River. There's also the NIU lagoon it's always relaxing to me to take a stroll trough there.
Another path I know of is after Annie Gildden and Lincoln Highway, away from campus, but just use precaution at night I believe that's where an unfortunate event happened a couple years ago. You''ll need to visit Ollie's Frozen Custard unless you're allergic, but best custard ever.
Other than that I think Sycamore has a minigolf and other games place.
But for bigger things it would probably be best to go to Rockford, Chicago, and even bigger cities.