Students locate 27 points on a coordinate plane and write down the letter at that point to reveal a secret phrase.

Here is another riddle, to review graphing points after you teach slope. My students always get confused that they go up first for slope, but over first for a coordinate.

https://www.teacherspayteachers.com/Product/Graphing-Coordinate-Pairs-Riddle-Shrek-Theme-4749006