Not sure if Lego is appropriate for a 1.5 year old (I would guess not), but growing up I didn't have many toys but one thing I played with the most, was a pathetic looking lego set.

I'm not talking the fancy stuff you get nowadays with wheels/pullels or where you can build the Millennium Falcon (I was so envious), but about 50 4x2 different coloured bricks and maybe 5 6x1 long ones.

But man, the amount of creativity that sparked in my younger cell was amazing. The amount of crudely built spaceships I built kept me entertained for hours.

Constraints breeds creativity. A lesson I still try and apply to by daily life/work.