cardboard tubes + box = hours of fun! We pieced together TP and paper towel tubes. Used marbles. The kiddo loved it and even still pulls it out to roll the marbles into a dump truck. He wont' let me toss any cardboard tubes now.
Playroom - Lego table with storage . sort of a DIY but looks like it would take all of 5 minutes to put this together (table from IKEA, storage bins and some tacky stuff to secure the Lego building plates)