Pinwheels are a great way to teach students about angles and different geometric shapes. Find some pinwheel templates online and ask the students to find the best angles for catching the wind.
You don't just have to use squares. You can make pinwheels out of lots of different shapes.
You could even hold competitions for the students to see who can come up with the pinwheel that catches the wind the best. This teaches innovation as well as basic mathematics.
Plus it is a fun way to spend an afternoon.