I read somewhere once that learning a language will have the brain form new connections and may even encourage an environment which promotes nerve cell growth in general. I don't need any encouragement. I love languages and spend at least half an hour a day teaching myself and I see a tutor once a week. Perhaps think about taking up a new language.
Mental challenges and tests are also supposed to help. I have not actually tried this but https://www.mymsmyway.com/mybraingames/index.php
was set up especially for us with MS.
If you don't already play chess you could think about doing so. I have fallen out of the habit of playing chess live with people from all around the world at http://www.chess.com/
. I keep meaning to get back in to it. You are matched with people of your ability whatever your level.