Have adventures. Step outside of your comfort zone. Don't lock yourself into a career or education until you've lived it up. I was at 1000+ concerts (literally) before I even started college. Those are the best memories of my life. Don't have kids. Just live it up.
When it comes to careers, like 1 out of every 50 people come out of college with the same career intent they went in with. Keep an open mind and use college to explore, not get married to a career path.
Your friends are going to slowly one by one start dropping off. You'll care less because you don't need as many when you're older. Everyone starts doing their own things. But pick one or two you can't go without and try to never forget to water that garden.
Your plans..... they mean jack shit. All of them. Plans are a basic guide, but life is going to throw you down so many detours your head will spin. Sometimes, you'll find yourself in places because of it, that you couldn't imagine life without. You're not in competition with anyone, and nothing that anyone else thinks about your life matters. Just live it in a way that you challenge yourself enough to not be bored, and have enough comfort as to negate the stress. Do your best.. nobody knows anything. You'll never feel like an adult. That's a myth.