In various senses, geometry will be the most thought-provoking topic. Since I entered junior high school, geometry topics have been increasing, and most of the topics in the senior high school entrance examination are related to geometry. As far as junior high school geometry is concerned, most of them need auxiliary lines more or less, no matter how difficult it is. Furthermore, the difficulty of many junior high school geometry problems lies in how to make auxiliary lines according to conditions, which is the most basic and training step. So in the second semester of the second grade, I learned a lot of geometry, and I couldn't find a difficult topic that didn't need auxiliary lines. To sum up, if you really want to exercise your thinking, you might as well think in another direction. You can search for some topics or books on the Internet. Here, I recommend some books, such as Cheats of Geometry Auxiliary Lines (Geometry Training and Competition with Answers) and New Methods of Training and Competition (Guangdong Olympic School, the textbook of the Middle School Attached to China Normal University).

hope this helps