This is a great problem solving activity involving similar triangles.
Divide your class into groups. Give each group a tape measure and a mirror (cheap ones, from the dollar store), and a list of tall objects in and around your school that they must determine the heights of. These could be things like flag poles, school roof, neighbouring buildings, etc… You might need to remind them that angle of incidence equals the angle of reflection (this isn’t physics class, after all).
Let the students figure out what to do with the tools they’ve been given. Essentially, they will place the mirror on the ground between them and the object. One student will position himself so that he can see the top of the object reflected in the mirror. The other students measure the distance from the mirror on the ground to the base of the object and from the mirror on the ground to the observing student’s feet. By measuring the height of the observing student’s eyes, they can use similar triangles to calculate the height of the object.
This website has a diagram of the activity, in case my instructions didn’t make any sense.