When my kids were little they loved playing with wooden building blocks with their Dad. He always encouraged them to use every block in each creation. As our collection of blocks grew, this became more and more of a challenge - especially since we would seek out the most obscure shapes and buy only 1 of each because it was so fun to watch him struggle with his innate sense for symmetry.
Lego creations and block castles are just two examples of fleeting triumphs. Why not create a small book of their best work?
How about creating a book or card for Father's Day?