Since this holiday is still more than a year away, and now that you have some starting points, why don't you spend your time till then reading:-

*through the NP website
*other websites you may find by searching on the park names
*books from your local and school libraries (there are bound to be some books on the US and/or its NPs)
*magazine stories on the area, especially old copies of National Geographic such as found in waiting rooms and also easily obtained through your local freecycle group. (Freecycle.org)
*watching relevant TV programs or videos on the subject
*plus any other resource which comes to mind

All the time having a note book handy to take notes. No doubt the park on which you have made the most notes, and which is not too far of your route, is likely to be the one which has caught most of your interest. Or you could find that the parks end up designing your route.

It is easy to go and see what others have liked and recommend. But it will not necessarily be that which interests you and your future spouse most. There simply is nothing like doing your own research.... driving your unique dream roadtrip.

Of course you are always welcome to come here to ask specific questions as they arise.

Lifey