Assuming the average person's utility function is concave with respect to money and given the current income distribution the simplest and highest utility change is to take a fixed amount from high income people and give it to low income people. This follows from simple economics as the people on the lower end of the distribution know best what it is they need. GiveDirectly is the charity that pioneers this exact scheme and that is why I donate to them.

On the other end of the spectrum, the high income countries, the best people could do is eat healthier and exercise more in terms of DALY won. Though efforts are made in that direction, they seem quite futile. Two other unexplored options are training in rationality (CFAR style) and training in methods of cognitive behavioural therapy (CBT). Obviously CFAR is the go to charity for rationality but what about the latter? How large could the effect be of training young adults in the mentioned techniques in terms of DALY seeing as mental health problems such as anxiety or depression are quite prevalent and increased psychological resilience may even help with the aformentioned goals - not to mention the intangible goods such as improved quality of life, less suffering from external events and 'better' relationships.