this is a great question. for me, it would be going to bed at a responsible time.
I actually had that thought yesterday. my younger self would be so sad about me cutting out precious video game time, but I literally can't focus on my job if I get tired halfway through the day.
but my younger self would not understand how lucky my life currently is, and that "sacrifices" need to be made to do the best that I can in life since many people do not have the opportunities I have. I got very lucky.