If you're just starting your career, side projects will have the biggest impact. Mastery comes from dedicated practice and side projects are just that. Make sure you're learning new tech, patterns, languages, whatever, but the more you learn now the faster you'll rank up.
If you're more senior, the biggest impacts will come from networking. Figure out where you want your career/life to go, then work towards that. Want to be CTO? Want to work at XYZ tech co? Want to start a business? Doesn't matter, make some friends and start networking.
I'm a big advocate for leveraging the energy of your youth, but make sure you're spending it wisely. There's more to life than working.