Meditate
Meditating is one of the best things you can do for your brain. By simply clearing your mind and calming your body, you can effectively cleanse your head of all the unwanted and negative stimulus that gets stuck up there. Meditating will make you sharper and help you maintain focus throughout the day.
Practice Ambidexterity
You can start doing this right now actually. The next time you brush your teeth, open a door or even take a sip of coffee, try doing it with your non-dominant hand. Creating these associations will build more neural pathways and help you become statistically more intelligent with barely any work.