Our oldest got a couple of the small proper-Lego Easy-To-Build sets when she was 2 1/2. That worked out very well.
Obviously we had to play together with her the entire time, and assist with the smallest pieces, but we quickly developed our own vocabulary for orienting parts along the different axes ("rotate" is around global z-axis, while "turn" is around global x- or y-axis, etc.) I'm positive it did wonders for her spatial cognitive abilities.
Especially the digger was a big hit, we must've built and rebuilt it dozens of times. Unfortunately it's no longer for sale:
Nowadays she's does a lot of freeplay with the bunnies and Elsa and stuff, but we still regularly do the teardown and rebuild of e.g. her Arctic Helicopter set:
Our oldest got a couple of the small proper-Lego Easy-To-Build sets when she was 2 1/2. That worked out very well.
Obviously we had to play together with her the entire time, and assist with the smallest pieces, but we quickly developed our own vocabulary for orienting parts along the different axes ("rotate" is around global z-axis, while "turn" is around global x- or y-axis, etc.) I'm positive it did wonders for her spatial cognitive abilities.
Especially the digger was a big hit, we must've built and rebuilt it dozens of times. Unfortunately it's no longer for sale:
https://shop.lego.com/en-US/Digger-10666
Nowadays she's does a lot of freeplay with the bunnies and Elsa and stuff, but we still regularly do the teardown and rebuild of e.g. her Arctic Helicopter set:
https://shop.lego.com/en-CA/Arctic-Helicrane-60034
FWIW, most of the Lego City series has that "join all the sets together" vibe going.