As far as GC NOTHING is available in the park, which I'm not surprised.
Sorry--don't know anything about the Holiday Inn. Even if you do make reservations there but are still interested in the staying in the GC park--keep checking Xanterra online and calling in case something opens up.
I have ressies just outside of Bryce NP for our stay there, but I really prefer to be in the park and I have been checking online everyday--I've noticed that things do open up (not for my dates of course!!) I keep checking Xanterra to see if El Tovar opens up, too.
Also, not sure if this is true for GC reservations, but a Xanterra reservationist for Yellowstone told me that a block of available reservations are set aside for online and that they have access to more availability when you actually call in. Granted that was for next summer--so more is available right now than for this summer.
Good Luck!
They have a shop and a restaurant and a museum. Yes, we also stayed at Sky Ranch lodge and Yavapai (the only onsite hotel available). Have a great trip.