Just a note from one who lives at sea level (NJ) . . . elevations at YNP start high and go higher. Expect to feel more tired, plan to drink a lot of water, wear sunblock (i was there in the fall and although I always wore long sleeves the back of my hands got very sun burned), and you might even have some difficulty sleeping. That helps take up the night time entertainment.

If you have a full moon, or close to that, seeing some of the sights again at night is worthwhile. Get up with the sun and have an early start to your day. Just think its a couple hours later "back east!"