Those are all great places! Grand Canyon is my most favorite place in the world 
A word of advice on Antelope Canyon if you’re going in the summer. Go early in the day, it’s hot and incredibly crowded later. Make your reservation ahead of time. Slide Rock State Park is pretty cool, although my friend did fall, split her lip and get a bad infection in it while we were there; I’m not sure how clean the water is since it’s so busy.
Sunset Crater National Monument / Wupatki National Monument are very close to Flagstaff and beautiful (and less crowded). We also love the Americana Hotel in Flag. It’s a newly opened and restored route 66 motel.
We always stop at Cameron Trading Post, and the hotel there is another one of my favorites. Definitely stay there if you can.
The south rim of the Grand Canyon will be crazy busy if you’re there in the summer, but I can give you some tips later on how to get away from the crowds.
I spent a lot of time just sitting in a car today, so this was a good thread to keep me occupied!
I don’t know if you’ve read this thread, but quite a few of us discussed a lot of cool places to shop and things to do here. Heading to AZ, yay! But need help