Papercraft, I just love it! It can be as simple, or as complicated as you want and all fantasy bases are covered.. Choose a fandom and you can find papercraft projects, for free. You can make lovely things for the nursery (I made some realistic looking birds, including a white owl). And older children can start making papercraft things for themselves, or with some help. You do the hard part and they take the credit ;-)
Cube papercraft is pretty easy and straightforward. Remember, you don't have to cut the folding parts perfectly. After all, they fold.. and you glue them. You do need them, so it's better to leave too much paper instead of cutting off too much.
You can choose any theme, find Cude papercraft models and print them at home. Use a thicker paper than regular paper for best results. If you really want to play big, print on A3 paper. Scissors are fine, but a hobby knife is good too.
I made a few Doctor Who-related Cubes. The Tardis, Rose, the Tenth Doctor and an Angel. Even though these are relatively simple, Papercraft is still great for fine motor skills and a nice way to spend a rainy autumn afternoon.
Not a Cube, but still a Tardis papercraft. I simply love it, because this Tardis has an interior and you can look inside it when it's finished. This link features the pattern (click on the picture) and also a tutorial.