This paper provides several mathematical classroom tasks based on the ideas of graph theory to encourage critical thinking in secondary school students. It is a theoretical investigation into the teaching of mathematics. Some graph theory concepts, including Complete graph, cycle, wheels, Shortest path, planar graph, tree, vertex and edge coloring, and Solid Geometry, can be used in secondary school to boost students' critical thinking abilities. The students should have fun doing these activities because the teacher may use them as a game in the classroom.