Learn a foreign language. I have been studying Japanese a bit, but have still a long way to go before I can watch anime without subtitles (which is my goal, Japanese writing would be too much I think). It sure forces me to think in different ways.
In general:
Physical exercise — Good for the brain as well as the body. No need to overdo it though.
Experience greenery — Good for mental well-being in general.
Meditate — Meditation is an exercise in acquiring mental focus, and being better able to focus can help with all mental tasks.