My son has a enormous bin full of lego that he just free builds with.

Every time we go by Lego Land when we're on a road trip we make a stop and gets to pick out a bunch of stuff from the "random brick wall" and he just adds a bunch of stuff to his pile.

He got a Minecraft set for christmas a couple years ago. I was the one that built it but he just uses the blocks and minifigs for whatever he wants now.