There is a little known place call the Grand Canyon Caverns. It is located on the History route 66 between Hackberry and Seligman. Very pretty place if you don’t mind being out in the middle of nowhere. They have a single open air room that you can rent by the night. It is on my bucket list to stay down there. It is around 600-800 dollars per night. But there are plenty of space to bring friends and split the cost.