I've heard that the key to mental longevity is to always keep your mind open to learning new things. One easy way is to adopt a few new words and use them in your daily conversations. Surely you don't want to sit around all day, flipping through a thesaurus, so let me introduce you to Save the Words.
This website is not only cool but pretty funny. Scroll around inside the white box, and words will jump out at you, just begging to be used in your next speech or convo. Click on one that looks interesting, get the definition, and sign up for an optional "pledge" that sends you a reminder to use the word as much as possible every day. It's a fun way to expand your vocab, and create those coveted knowledge wrinkles in your brain!