Some of the most simple things one can do to maintain good health include at least 8 glasses of water per day. This really is a minimum requirement. More is better especially in warm weather.
Performing some kind of movement (aka exercise) is also something that should be carried out by healthy or want to be healthy individuals. Exercise does not mean formal and expensive, hard to do routines. There is no end to ways to incorporate daily activities and find something that is a great fit for your fitness level, age or time availability.
Then there is the healthy eating routine to maintain normal weight. It is easy to get confused with all the diets out there. If one has any medical issues, it just further complicates things. It may not be a bad idea to consult a dietitian at least in the beginning if one is not well versed in this area.
The importance of adequate and good sleep is often overlooked. The long term effects of inadequate sleep are numerous and include changes in hormone levels, poor performance at work. Even short term lack of sleep has deleterious effects on psychomotor activity.
I want to include the maintenance of healthy interpersonal relationships as an aspect of one's overall need. This affects your emotional wellness and impacts your ability to be happy and to thrive.
Sometimes healthy people ignore the spiritual connection. I dare say that is just as important as the diet and exercise.
Every little bit counts. Tiny changes amount to visible outcomes if carried out consistently.