There are several miles on each side of the Mississippi River, up towards the northern part of IL and Iowa that are far from being flat. Once you get about 1/2 way across IL, then flat pretty much describes IL How far north in IL the road trip will be makes a big difference.
Not sure where Cob lives, so I just picked Des Moines IA to Ober IN. Map Quest says just under 6 hours on the interstate. Lots of flat fields on that route, pretty much misses any scenic country. A 3 hour detour farther north (on mostly 4 lane highways) could take you far enough to see a couple hours of decent scenery. You would be up in my area then, I could take you off the major 4 lane highways along that path that would really show you some pretty country, but that too would probably add another hour to your trip. Probably not the road trip you have in mind.
I'm jealous, the only trip I'm planning is to the Dr. and hoping the snow misses us so I don't have to use the tractor snowblower again. We did the snowbird deal for 6 years, rethinking that decision this year, your winter beats our summer in Northern Wisconsin. I usually have my hobby cars put away sometime in October, get them out in April/May. At a car show out there, I was in a conversation with a local, he moved out there full time from Minnesota, I asked what about the hot summers, his reply, I've never had to shovel sunshine 1X, so we got used to it as best as we can and thank God for AC. We went up Mt Lemon, very scenic, nice restaurant at the top and when coming back down take one of the small blacktop roads to the right and you'll see some beautiful homes tucked in the mountain side.