The Clue to the Location
From what they have shown in this episode, it has a large spot in front with a good view of the Great Valley and has a small cave as well.
It is during this episode while Ruby is there that an earthshake happens that makes a part of the area that Ruby likes to sit breaks off and falls onto the enterance which she is trapped until being rescued within the cave. Though since the gang can stand in front of the cave entrance, it seems that Ruby could still go here to use it, though the area in front of the cave is a little smaller after this episode.
Ruby's Thinking Place has appeared in the third episode of the TV series called 'The Star Day Celebration' which Ruby apparently goes in if she wants to be alone to think. The episode had the location with a large spot in front with good views of the Great Valley and has a small cave as well.
An earthshake happened, which caused one part of the area that Ruby likes to sit on, breaks off and slams itself on the enterance, getting her trapped. The gang arrive to rescue Ruby, and they all return home.