Remember making angles with cardstock and brads to teach acute, obtuse and right angles? I did it with students just last year. Then I learned about AngLegs, adjustable angle manipulatives made from colored plastic that can snap together, extend and retract. They come with snap-on View Thru protractors as well. These are amazing! Students can build angles, then attach them to build regular and irregular polygons. They can manipulate the sliding pieces to see the relationship between the length of sides and the angle measures, explore height of polygons, calculate area and perimeter, and find center points. These are fantastic for showing congruency and similarity in shapes.
AngLegs are advertised for grades K-8 but Amazon reviewers report using them even with preschool students for discovery based learning and to practice making shapes. Other teachers report using them in high school geometry class (see links in resource section). They are a truly versatile manipulative and are, in my opinion, a must for elementary school hands on geometry.
Resources for Teaching With AngLegs
Free AngLegs activity cards for grades K-8 from Learning Resources
Guided Discovery of AngLegs for young children from Playdiscoverlearn
Super Secondary Manipulatives: AngLegs for high school geometry
Analyzing Triangle Congruence With AngLegs for high school