This is a Geoboard: (well my homemade version)
I found this idea on a homeschooling blog (can’t find the page again to credit them though). Basically, you use a piece of sanded wood, paint (or spray paint), mark a series of dots on it using a ruler or grid paper. I made my dots 1.5″ apart in all directions. Then nail tacks into the wood. And tada, you have a geoboard.
Kids can then use rubber bands or stretchy ponytail holders to create shapes on the tacks.
Haddie has now learned what a rhombus and trapezoid are (and so have I) from the geoboard (and Wikipedia!).
She enjoys playing with it and I used it for tray time on the last homeschool day. The boys really enjoyed it once they figured it out. I also brought it on our Atlanta trip and it kept the kids busy for a while.
And here’s Haddie’s own creation of squares: