Anyone thinking about driving Old 66 should pretty quickly come to the realization that it could become a summer long excursion. Old 66 exists mostly in bits and pieces and the good guide books will provide you with way too many places to stop and check them all out - unless you are retired and really want to "see" 66.
I would suggest you first do your research and then plan out some stretches of highway that interest you and along those areas determine the special places not to be missed.
And your stretches of highway and your spacial places will probably be completely different from what somebody else might set out to visit along Old 66. There is that much to see and do.
EDIT: As an example of the amount of time that could be spent along Old 66, I just grabbed my copy of Richard and Sherry Mangum's 2001 book "Route 66 Across Arizona" - somewhat of a classic and now out of print. It has almost 50 entries in the table of contents and almost 300 in the index. This is just for Arizona - though I suppose Arizona is especially rich in Old 66 places of special interest. And the state has some pretty nice long stretches of the old road to follow.