Great idea for lego storage! Instead of flat surface, add lego mats. Creative Solutions for Small Space Play: Underbed Play Table with Drawers by Great Little Trading Co. (does not ship to U. but think of the DIY possibilities)
DIY Pattern Building Blocks: An Awesome STEM Activity for Kids
Life size Lincoln Logs made out of pool noodles~ 15 pool noodles from the dollar store, cut in half, cut notches out easily, with scissors = hours and hours of fun playtime! These were my favorite toy as a kid, love the life size!