This is about as easy as it gets and occupied my kids for a very long time. They get this out a lot!
All you need it a pool noodle, some marbles and something to catch them in (or not). I used a roasting pan because the kids were able to hear the marble "ting" at the bottom. My two created marble races, and discovered which marbles fit down the noodle, which ones got stuck and which ones went fast! So fun, cheap, easy and all sorts of skills being used. Some of the skills are cause and effect, small motor, cooperation, language development, comparing and contrasting, hand/eye coordination, sensory, shapes, oh so many skills!