Easye that's a good question.
I wish I had gone to collage right after high school into a field that would earn me more money and have a fuller work experience than I did. Instead I went into the army thinking I would do that for 20 years, but after a few knew it was not for me. The life experience was good, but the loss in time maybe not so good. I am in the medical field now, and while it pays well now after 32 years, it wasn't always that way, and I can't say that I love doing what I do. I am good at it, but don't yearn to go to work. Counting down the days.
I wish I had taken more care of my body. There's a youtube video of Warren Buffett that talks about getting a car at graduation, free, but the only car you will ever get. Then talks about how your body is like that. Watch it.
I wish I have traveled more. I traveled a lot as a travel nurse in the states, but it was mostly work. Not for pleasure.
I wish I had learned another language. I had the opportunity twice. Once in grade school, with Ukranian, and fought learning it, once after with spanish which I know some, and plan to learn more in my sailboat travels.
I have done a lot of backpacking and outdoors stuff. I gave me a lot of pleasure. I wish I had done more.
I wish someone had taught me the true meaning of money at a early age. My parents had no clue, school is a big failure in that, the internet did not exist. I learned slowly and painfully, it cost me more than I can say.
So, take care of your body, learn something new every day, read a lot, travel, learn the value of money and how to earn it, keep it and grow it, learn to be self sufficent and live below your means. Don't be taken in by people trying to sell you stuff you don't need. Live for the moment, but plan for the future. Always understand that our actions have consequences, so think before you act. Don't jump into relationships, but carefully think it out before you commit to something that might set you back a decade or more......
Hope that helps.
Bob.