You can make your idle money work for you by investing it in the game. The key to making your idle money work for you is knowing how to use the power of other players and resources. You will need to be able to produce a high-quality product that people want, or find a way to produce something that is needed in the game and get paid for it.
Another option would be finding ways to increase income through farming, fishing, or crafting, which are all great ways of earning an income while playing this game.