We were looking for an overnight stay while traveling through Idaho.  We didn't arrive until 5pm and were certain the campground would be full.  Much to our surprise it was not full but we were not told how many sites were available so pretty much took the first site we saw.  It was a huge site for our little rig but we grabbed it anyway.  We enjoyed visiting the park in the evening and having it almost entirely by ourselves.  We also had a little time in the morning to catch some areas we missed the night before and also had this time to ourselves.  Had we not stayed in the park I'm not sure we would have had time to visit on our way (20miles) to the next nearest campground.  The park is fascinating and worth the drive.