In his later years (and as dementia was beginning to set in) my dad spontaneously decided to start building acoustic guitars in all sorts of shapes and sizes, trying out various weird ideas he had. He hugely enjoyed it and it's left behind some interesting heirlooms and a legacy that he otherwise never expected to leave. A piano is at a different scale (pun intended) but if you have a modicum of practicality, you can build something like that with enough time.