Be seen. Volunteer for on-campus events that are organized by the university (find the person running something, and ask them directly if there's anything you can do to help them).

Always ask "Is there anything else I can do to help" after completing a task. Always be the first to start volunteering (show up early) and the last person to stop volunteering (stay late).

If you're a person of good character and genuine spirit, people will notice and talk about you. People will ask your help with bigger things. Repeat.

I think the answer will differ wildly for different people. What I wish I had done:

- Pay at least a little attention to nutrition.

- Be more willing to do things by myself; there were certain activities, for which I had a vague impression you were supposed to wait until other people invited you to go with them.

- Be less afraid of upsetting professors and other authority figures. (And I say this as someone who is a professor now!)

- Try less hard to weasel my way into the "in" crowd. (And by "less hard" I don't mean "not at all"; these efforts were worthwhile to some extent.) Conversely, there were all sorts of interesting people who weren't "popular"; try to make friends with them, or even ask them out on dates.

- Somehow found a way to relax more, although that is always much easier said than done.

* Try new things, join groups, stick your neck out. Don't find out you really prefer [X] or [Y] when you're 40, figure it out when you're 19. College fencing, for example, opened a whole world for me, like SCA, blacksmithing, etc. It's always going to be awkward joining/hanging out w/ a new group, just be patient and keep going.

* Spend as little time in your dorm as possible. Be out with people, read a book in the library or student union, join an intermural team, go to a party, whatever. Don't fall into the Weed-And-Vidya-Game pattern that college types tend to have. I suspect one of the reasons many dorms make students share a room is to force this.

* Be willing to say no to a course of study, or even to college. It's not for everyone, and I hate that we force people into it even if they don't have the money/interest/aptitude for it. The best-off people I know are CS grads, but several grads are not making great cash. Meanwhile I know (hired) electricians making north of six-figures, and I know a couple plumbers who are doing quite well.

* Most of the couples who got married after college are now divorced; most of the couples I knew in college split before graduation. Some are still a thing and happy, but the takeaway is don't limit yourself because of relationships. Date around if you can.