Been over most of Route 66 from Chicago to Flagstaff. Some of it a few times, some of it many times. You can find almost 200 miles in Oklahoma, more than 120 in Missouri, all of it in Kansas.
Most of the 1960s alignment was closely followed by Interstates 55 and 44, then I-40 west from OKC. Through some areas, the Interstate is on top of old alignments, but because 66 went through towns the Interstate bypasses, Route 66 ends up being the "business route."
A lot of attractions to take advantage of the nostalgia and mystique.
Get a good guide, decide what you want to see, what sections you want to follow, on what alignments (routing changed a lot in some places over 30 years, e.g. it once went through Santa Fe and later Santa Rosa). Also, what you want to bypass on Interstate to save time: 20 minutes vs two or three hours to get through St Louis?