With one exception (I'll get to that in a minute) I agree with the other replies about getting off the Super Slab. A couple of memorable drives come to mind from our three cross-country voyages:
1) Rt. 10 from Walsenberg CO to La Junta CO. A very easy, scenic drive and at the right time of year you'll pass through huge fields of gorgeous lavender. I'd be surprised if you didn't have the road to yourself the whole way - this is a well-kept secret!
2) Rt. 12 from Syringa ID in to Montana via Lolo Pass. While maybe not as much "fun" as the above drive, it is very scenic (mountain/woods). Just be sure your fuel tank is full and your rig is in good running order. Getting stuck behind slow-moving commercial vehicles here will take some of the fun out of it.
3) I-80 in PA from the Rt. 15 interchange west to Rt. 144/Snowshoe, PA. For those who think Pennsylvania is nothing but abandoned coal mines and steel mills (or maybe Philly/Harrisburg/Pittsburgh traffic congestion) this stretch of interstate comes as quite a shock.
Enjoy & safe travels.