Children of all ages are fascinated by marble runs, and making their own can keep them occupied for hours. Try...
The junk modelling marble run. Gather together as many cardboard tubes, packets, foil and polystyrene trays, etc, as you can find, together with a roll of masking tape and strong pair of scissors. Then set a challenge - for example creating a run that will take a marble from the table, across a chair and down to the floor, or down a small flight of stairs.
The magnetic marble run. Instead of masking tape, put sticky magnets on to the back of your cardboard tubes and stick them to a fridge, dishwasher or cold radiator for a marble run that can be endlessly reconfigured.
The cardboard box marble maze. Find a shallow cardboard box and use drinking straws, bottle tops, corks, matchboxes and more, stuck on with blu-tack or masking tape, to create a maze to roll a marble around by tilting and tipping. Use a small foil pie case, lid or similar to make a 'goal' to get the marble into, then race against the clock to see who can get the marble around the maze in the fastest time.
The Lego marble run. If you have plenty of Lego or Duplo, try creating a marble run or maze out of that too.