Perhaps wooden building blocks with different colours and shapes? You can't go wrong with these - worst case they'll learn category theory ;) But to be fair (and completely serious now) I would not get too obsessed about that. Kids have a way of showing you what they find interesting at any given moment and I think it's best to just go with the flow without planning for specific outcomes.
My first-grade son was playing at 'DAP time' where they checked out a kit (blocks, string, whatever) and did whatever they wanted with it for 30 minutes. He had the box of keys.
Now, nobody checked out the box of keys - it was boring. Nothing you could build; no clear categories (too many kinds of old keys of too many shades of color). But the teacher noticed him taking them one at a time, dropping them over his shoulder, picking them up again and putting them in a row on the table.
She didn't disturb him - that was a rule of DAP time, nobody gets to interrupt you. But when he was done and had put them away, she asked "What did you find to do with the keys?"
He answered "I was putting them in order by sound". See, he was constructing a scale of sorts, by the sound they made when they hit the floor.
Wasn't another year when we discovered he had perfect pitch. Went on to master the piano and the cello before junior high; played in every school orchestra and event. He was playing professionally in High School (City orchestras; Orchestra Iowa) and competing at the national level.
Anyway, never underestimate the use of simple toys.
There's these very nice swiss wooden blocks that have pathways for marbles carved into them.They're a lot of fun, and dead simple to put together, but also quite expensive, at about 150EUR for a set of 50. I had some of them as a kid, and now I'm a competitive minecraft player, so there's that.
