Action. Things that move. Preferably, things he can make move.
That Santa -- he's a smart guy. He brought a Thames & Kosmos Air + Water vehicle set for Christmas. Boy, do they make some great sets! (I imagine we'll have a few of these when it's all said and done.)
We've built a few of the models so far. This was the "tank," and it got great reviews!
I love this activity as an illustration of how much (academic) learning is happening using a project-based learning approach. And to be clear: this isn't the real point. But I think it's worth noting. He's just "playing." He loves to make things go.
"I want to build this, Mama!"
"Where does this piece go?"
"Do we have two big circles?"
"If we snap all of these together they make a chain that connects the gears."
"I can do it!"
"We need a ruler!"
"Fifty pumps to make it go!"
"When I pump it, the water goes from this tank to this bottle."
"We need signs!"