Start with the general idea
First, get yourself an atlas or a set of maps for the states along the way -- study them and see what roads look like they might be interesting to you. Look for roads that pass lakes, or follow rivers or streams! Take your fishing gear and ... Oh, never mind, I got sidetracked there for a minute.
You might want to do a combination of interstate highways (I-70 or I-40, for example) and federal two-lane roads, for a little variety. If you use the two-lane roads, remember that the more towns found along them, the longer they take to drive -- you have to slow down when going through towns, and this costs you more time than you'd think. Sometimes, those little places can be the best part of the trip though, so don't avoid doing SOME of that kind of driving.
Another planning tool is MS Streets and Trips software -- worth its weight in gold (my opinion) for playing with routing, figuring mileages and times, etc. Decide how many miles you want to cover each day, and don't overdo it -- or, simply keep an overall timeframe in mind, and drive each day with a "see how far I can get today, and what will I find along the way" mentality. Good luck and have fun planning! Bob