Geoboards: How To Make and Teach
Everything from how to make them, to an overview of all the topics they can be used for, and materials on specific topics like Polygons, Graphing on the Cartesian plane, Pythagoras Theorem, etc.

Compass and Straight Edge (and Angles)
For centuries, an essential part of Mathematics education was a study of The Elements by Euclid, and learning how to use a compass and straight edge. Many mathematicians, like Gauss, were masters of the art. So using a compass and straight edge starts with a study of some of Euclid’s propositions, and goes on with some other interesting constructions.
Probability (and Statistics)
Hands on materials like coins, cards, dice and blocks make a tricky topic like Probability more fun, more challenging, and more understandable.
Number Theory
Known as the Queen of Mathematics , number theory goes back to ancient times and has strong connections to Geometry.  It has many interesting problems, and modern applications in computing.
Other Topics
Other Lower Secondary Maths topics not covered in the above categories.