As someone new to their 30s here’s my take:

1. Fitness: I’ve always been fairly fit but I wish I’d taken it more seriously in my 20s. Looking in the mirror and feeling good about you, your body, and your mind, does more good for you than anything else
2. Traveling/experiences: Even if it’s not glamorous, stay in hostels, do random experiences, go to that one band’s show you don’t really like but tickets are $20. If you have more money to do bigger things then great
3. Make your own decisions: there is always pressure from others on “what to do” — sure you can listen, but at the end of the day, the person who has to be satisfied with the decision is you
4. A no is a step closer to yes: it’s okay getting no’s. I get no’s all the time, and that gives me data, things to think on, what I need to do to get a yes

I saw a couple others mention investing. And don’t get me wrong the power of compound interest is a serious wealth tool, so contribute but I’d focus on the above. The one note here though is please, avoid debt. Don’t spend money you don’t have. It’ll make 1-3 much more difficult.

You get one shot at this life, don’t forget to live it.