My current top-of-the-head list.
Exercise is good, sitting is bad, moderate consumption of alcohol is probably good, smoking is bad, fresh vegetables are good, refined sugar is bad, family and friends are good, stress and disrupted sleep are bad. You may have noticed a trend, which is that all of these (except for the alcohol one, maybe) , are thoroughly mainstream. If this trend represents the state of research, I'd suggest a national public health agency's website for the really good interventions.
Also on the "mainstream/obvious list":
Being obese is bad. Being overweight probably bad. Being underweight is probably also bad. Vitamin D good. Getting enough micronutrients in general good. Excessive red meat consumption probably bad. Excessive processed meat consumption bad. Laughter good.
(That's all I can think of off the top of my head that's not yet been mentioned.)
Edit: Oh! Forgot one. Sunburn bad.
Dying or becoming severely physically/mentally ill is very likely going to significantly lower the output of your utility function, so it would probably be a very bad idea to ignore the low-hanging resources which can significantly extend the time for which you are alive and well. I have attempted to search LessWrong for a list of such resources, and haven't been able to find one.
Are there any books, websites, or posts that contain significantly low-hanging fruit in this area? If so, please list them in the comments below.