I-40 is your target route as Jim said. Beyond the Grand Canyon, accessible either from Williams or Flagstaff, there are a number of side trips worth while.
1. Santa Fe -- uber quaint but fun, great for Indian crafts. There are accessible pueblos in the area.
2. Petrified Forest -- just off the freeway, interesting but only for a few hours.
3. Meteor Crater -- same as 2 but intrinsically more impressive, that was one heckofa bang that dug that hole.
4. Canyon De Chelly -- the memories of a life time. North of I-40 on US 191. A vertical walled sandstone canyon with ancient cliff dwelling ruins and current Navajo ranchs. A 4WD trip either your own with a hired Navajo guide or a 4WD tour run by the local resort. Half your driving is on a hardbottomed river about 6 inches to a foot deep Allow a day's layover for this. Nice RV CG run by the Navajos.
5. Scenic tour of the Navajo and Hopi reservations on state 264 from Ganado to Tuba City and then into Flagstaff. Couple this with the Canyon de Chelly trip. You will gain the impression that you are in a foreign country. You are.
6. Flagstaff. Worth at least another layover. Neat and tidy city grown up around a little old western downtown. (Hollbrook and Winslow are a bit grotty.) I like it better than Williams for a Grand Canyon base of operations.
And, of course, the Grand Canyon itself. Do get your reservations in if you want to camp in or near the park.