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.
DIY Sand and Rock Box by FSPDT. Gdjk -> made this with toilet rolls (aka tunnels) and moon sand instead of play sand. My son loved playing with it. The moon sand cleans up nicely indoors but play sand will be easier to play with when using the small cars.