Saw these style boards on Oriental Trading for use as geometry boards. Wondered a diy hack. Use glue on bottom circle of thumbtack to secure it from being plucked out by little fingers. Sowdering About in Seattle: Toddler Busy Boxes
Learning basic sewing with a polystyrene plate, yarn, + plastic needle. I remember doing this with craft mesh when I was a kid and then graduating to sewing buttons by hand onto little pillows and pockets.