Get a load of this: pipe cleaners and straws. That's all you need to make triangles, quadrilaterals and polygons. I did this back when I had my fifth grade class. There are so many rules in geometry that we can take for granted. For instance, I asked them to define a triangle for me (after they joined three straws together). They said 'a plane shape with 3 straight sides'. So using that definition, I gave them a non-example:
An open plane shape with three straight sides. Is it a triangle? No. So this means that the three angles must be important to mention. In fact they were important enough to give the triangle its name.
We explored every plane shape we could make. And it challenged the students to pay attention to detail when generating their own definitions. It was a fun experience. Of course, you can try it with your class but you do not have to stick to straws only. Craft shops sell the sticks that you use to make mini flags, not the dowel sticks but hollow plastic ones. There are also the sticks to which we attach balloons. They are more durable than straws, just use shears or knife to cut it into shorter lengths and use eve shorter lengths of pipe cleaners (chenille stems) to make the corners. By the way, they also interlock so that you can make cubes, cuboids, pyramids and prisms with them too.