I found Rehgehstoy to be really fun to play with, and really interesting, so to keep me focused while playing and figuring out how the system worked.
I like these kind of toys systems that are a joy to discover and keep the player engaded.
I think that in this game this toy-system works so well because having a blank canvas, an instrument to make something that has personal results, makes it particularly satisfactory and keeps the player curiosity.
I liked a lot having the library there, it really gave a subtle narrative element to the game, like "you are in your laboratory and you can consult books by ancient sages and try to learn from them / look to your past experiments"