A few weeks while cleaning in the bedroom and talking on the phone, M. kept running in and out telling me he was making a rectangle and a triangle. I'm ashamed to say I didn't pay him much attention just telling him, "That's good!" and just assumed he was playing with blocks.
When I walked into his room a bit later, I found he'd made a makeshift Geoboard out of his dresser! Guess it's time to pull out the real thing!