One of Lucas’ favorite toys are these magnets, i think the brand is Magix. they are simple shapes, just squares and triangles. They have small magnets along the edge that allow them to be assembled in to a variety of structures like buildings, bridges and rocket ships.
one of the most fascinating things that Lucas does with these are how he figures out how to put stuff together on his own then finds ways to integrate play with other toys like cars, Paw Patrol and Olaf figures. the bridge was a recent addition to his designs. We used to just make houses, barns and rocketships.
I wish i could find more of these toys in shops, they are pretty cheap and rare.