I would suggest poking around on this website. There are many books and other websites recommended that would be good to explore in order to find places to visit. One of my favorites is Roadside America with quirky stops like the largest ball of twine, etc.. Other books of interest might be those that direct you to factory tours, gravesites of famous people, etc.

I hope you and your grandparents have a terrific time! What a wonderful way to enjoy each other and get to know each other better. It will surely give you some wonderful memories to treasure.

You might want to video some of the trip for a way to share it with future generations.