If you want your efforts to multiply, you'll need to do work that:
1. Lives on after you've stopped doing it
2. Matters and affects more than just you
3. Can be understood without you having to explain it every time someone new sees it

This way you can do a lot of work now and over time, work less (if you choose) or move on to something different. You don't want to be trading your time and effort -- very finite resources -- for money. In the end you will always lose. The ROI on that only benefits the "money", and by that I mean the person/people supplying it in exchange for your life.
Build something that works on its own and you become the money.