I had asked a open question to my 7 year old son and asked him to make a rectangle width 48 pieces of snapping cubes. He came up with a lots of fun ways and really enjoyed it.
Would anyone have suggestions for open challenges with the snap cubes?
You can start from multiplication towers: there are greatly many ways of doing it. Then you can open the task even more by modeling other "two-input function machines" with towers. This way, you can build surfaces not only for multiplication, but also for addition, subtraction, or any operation your child makes up. That chapter in the "Moebius Noodles" book has about a dozen ideas.