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.
Old/spare guttering lying around? Use it to make your own ball/car/water ramp! A huge hit with my children and took 5 mins and was FREE to construct! Attach guttering lengths to your fence with gutter clips (do this at a gradient/staggered so objects can run down the pipes) Provide buckets with water/jugs/balls/cars and let the fun commence!
DIY Light Table for Preschool. there's a link at the end for a more sophisticated one. i ended up just using what i had: christmas lights, black construction paper & tape for the sides, and waxed paper & tape under the lid. did the trick for me:)