Should you focus on one thing?

Question of the day after discussion with my wife – how to focus on one thing (which is a logical thing to do)? When you have a lot of ideas and some of them you know would be more fun to work on. And some you know would be more beneficial (financially) to work on in a longer term. How to choose one? And also, you know that writing, exercising, doing some leisure activities, etc., would also beneficial to you but would take your valuable time. So, should you sacrifice those activities for the sake of that one main goal?