Anything that gives you joy or happiness that is entirely for yourself. I have always struggled with self care because it always felt selfish... which is what it is supposed to be. Taking something for yourself can feel awkward at first while you look at all the things that need to be done or the expectations others may have for you. I have always struggled with the need to be the perfect father, husband, son, and employee. With this expectation of myself I left little room for self care and became frustrated, depressed and disengaged in all aspects of my life. All from a lack of self care.
Self care can come in the form of reading, exercise, or just being by yourself. I find myself taking ten minutes here and there to read and write throughout the day to provide myself with a mental break and a chance to analyze my life. I spend my commute in the morning reflecting on what's going on in my life and write them in a journal as soon as I get to my desk. Later I may take some time to write more about what's on my mind in a more formalized setting (like this). When I go home I again reflect on my day and let them settle in. I then take the time to talk to my kids to hear about their days and enjoy the time I have with them. All of this provides me with a level of self care. This could be different for you but the need is always there to be able to find out how to take some time for yourself.
Finally being good to yourself requires a certain level of improvement. A house over time requires updates otherwise it will fall down around you. This comes at a cost in time, energy, or money. Obviously this seems to be the opposite of self care. But in a way it's complimentary.
Plants will stop growing and eventually die if not transplanting. However transplanting will put a plant into shock and require more energy to rebuild. In the same way taking the time to improve yourself can require a little more from yourself but in the end will help you thrive. When I say improve yourself it can be anything from eating healthier to getting a degree. The point is to have an idea to work towards and once you've succeeded establish another goal.
Every year my brother-in-law creates a list of goals based on [Zig Ziglar's wheel of life] (https://www.ziglar.com/articles/the-wheel-of-life/). It's in an attempt to improve areas of your life to create a better balance. I sort of do this as a tradition at the beginning of the year. But I find that the commitment can be a little much so I tend to have broad goals like "Write more" or "Learn a new programming language" (falling into the areas of mental and career on the wheel). Then each month I try to come up with a list of things I can do each month to improve on those areas. Which may be things like "write four blog posts" or "create a Python API". That way I can iterate and grow towards my goals.
Go and Be Good To Yourself.
When you look at your life you often spend it living for others, which is great! But sometimes it's necessary to stand back and look at you life. Are things bringing you joy? Do you feel fulfilled? If not then it's important to look at what does bring you joy or fulfillment and try to make it a priority. Take the time that you require because the only way you can help others is to be good to yourself.