join a group
Many of our higher cognitive functions have evolved to facilitate social interactions, so it should be no surprise that socializing is good for your brain. Various studies have shown the value of family, friendships and community in maintaining health, happiness and longevity.
Some of us already have enough of this in our lives and perhaps would benefit more from some down time to relax, meditate or otherwise work on ourselves. But for the rest of us, making some new friends or joining a group can be fun & challenging in addition to good exercise for our brains. Few things can stimulate our minds in so many different ways as other people.
For a little more information on the subject, check out these related posts:
Looking for a group to join in your area? MeetUp.com is an excellent resource to find clubs or organizations to join – or to start one of your own. Craigslist.org is also a good option to find people to connect with. Likewise for Facebook.