"Tsumiki" chalk building blocks by a Japanese company. This is such a cool idea for kids to design there toy with chalk and then redo it over and over again. I bet you could chalkboard paint wooden blocks and DIY.
Shaving Cream Bathtub Paint - kids love it and it cleans the tub! and so easy to make. Oh I'm gonna do this.Shaving Cream Bathtub Paint - Use a muffin tin with shaving cream squirted into each cup & mixed with different colors of food coloring.
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)