Esoteric programming languages are languages that have been created not to be efficient or useful for a task, but to be fun. They often involve a gimmick that makes them weird or difficult to use. I believe the most well known esoteric languages is brainfuck, which uses only 8 characters, but is Turing complete. There are many things written about it already, and many resources to learn from. *bla blah.
Two other esoteric languages I like are Taxi and piet. The first requires you to create a schedule for picking up passengers in a small town and the latter asks you to create abstract 2D art. There’s a lot of variety to find.