Expanding on #3:
Team sports & activities can increase not just mental acuity, but social acuity as well. Being academically intelligent will get you far, but being socially & emotionally intelligent as well will get you even further.
Also, martial arts, dancing, and any kind of activity that requires fine coordination can significantly build new neural patterns. If you enjoy doing them, then the overall mental health you gain is a total bonus.
#7:
Learning a new language and/or travel outside of your country. Different languages and cultures can actually affect the way you perceive the world in subtle (and sometimes not-so-subtle) ways. They expand your understanding & view of the world, thereby allowing you to see an issue from multiple viewpoints.
A quick example is national and global politics. I don't really care for politicians, but I try to follow the shifts in the world climate as much as I can. And I'm always impressed by how my European friends seem to have such a keen understanding of US politics, while I'm scratching my head about European politics.
#8:
Foster many interests. If you have a random interest in birds, feed it. Read books on birds. Go bird watching. Join a bird watching group.
One of the reasons people are cited as geniuses is because they are able to engage in lateral thinking. They can see patterns in seemingly unrelated topics. This is a powerful mental mechanism that can help in all facets of life - especially in solving difficult problems.
#9:
Engage in critical thinking. Practice the art of asking "Why?" Don't always accept what you see and hear at face value. Question it, probe behind it, and get at the underlying truth.
The news media is a great place to practice this. I used to work in this industry and even studied subjects like communication, propaganda, and marketing in college.
Allow your curiosity to take over as you read the news or watch a TV show. If something doesn't make sense, try searching for more information about it online. Read about it until you've satisfied that itch of curiosity.
One word of caution here: this can be taken too far. I think it's a fine practice to think critically about the declarations people tend to make. I have many friends who say things that seem to be full of questionable assumptions. I want nothing more than to question their statements and challenge them. But doing so can easily cross the line into obnoxiousness. For friends who like to debate, I'll jump right into it. But I pick those battles carefully. Some can take it, some can't.
I hope this helps. It's great that you're asking this question. Wish I had asked it when I was 19. Good luck!