That IS a big challenge! This late in the game, a good approach might be to suggest to the students to create some sort of pegboard 3-sided cube or even an upside down "T" made of pegboard. Suggest twist ties or pegboard hooks to put toy pieces (from their own closets, from the thrift store), PVC scraps, cardboard cutouts on to accomplish tasks. Work with students to prioritize points. One hint that might help is for them to make some "place holders" such as a ramp or tube that a marble can roll down. Put these place holders between completed tasks. As the students add tasks to the device, they can put them where the place holders are. This year's challenges are doable but hard and a pair of students that figures out the start task, the end task and perhaps one in the middle starting this late in the season will be quite busy indeed.