As to your request for the flattest route (somewhat subjective, since you still have to go over mountains in the beginning), here's a route that I've driven and can attest that it is boringly flat...
I-40 all the way west to Nashville. I-24 from Nashville through Kentucky into Illinois. Pick up I-57 in Illinois and take that north to I-74 in Chanpaign/Urbana area, and take that west briefly to Bloomington, where you can take I-39 north all the way to near Rhinelander, WI; I-39 becomes US 51 up to Ironwood MI, where you can take US 2 over to Superior.
Mountains early on in this route, from Raleigh to Knoxville and then some hills between Knoxville and Nashville, but flat from there on, especially Illinois and Wisconsin. Aside from Nashville, most major urban areas are bypassed, and the few smaller ones that you'll encounter are very doable/easy to get around.
Did this route in reverse, but to Greenville, SC instead of Raleigh NC.
If this is a route that interests you, then you could Google sights along the way that you might want to visit. One recommendation I would make, up north, is to visit the area around Ashland/Washburn/Bayfield, WI. Short ferry ride over to Madeline Island, which is a fun area to explore. Be sure to take insect repellent, the black flies are fierce there (or were when we were there).
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