Use the brain for your health and to create good vibes. Learn new things. Laugh. Hang out with friends. Talk about ideas, feelings... Draw Imagine. Do all the stuff that makes kids happy. In general, kids have the healthiest brains (well I don't have proof of that really), so learn from them because they know how to learn best. Here's a bunch of exercises that might be of interest. You can think of them as brain workouts, daydreams, meditations,....
Recall preceding days
One I just learned: every day, remember five things that happened for each of the five preceding days. This will sharpen your memory. First start with simple things, then once you can do that quickly, remember the pictures and smells and sounds and feelings, etc associated with each.
Hear the song in your head
Remember a song as well as you can, the words, the melody, the different parts to the song in its entirety.
Picture the Mona Lisa
Try to remember all you can about a famous painting or work of art.
Create a work of art
Imagine there's a giant canvas in your mind and a paintbrush. Paint a picture in your head, brush stroke for brush stroke. You can change colors, too. And add things other than paint. You can make a sculpture.
Think of something you know really well. That you can picture in you head. Say a basketball or an iPod or an apple. Picture the item in your head. Now spin it around, then spin around it. Go closer and farther away. Imagine it spouts wings and flies away. And imagine there are two more and they all have wings or feet, and are flying or walking around. Etc...
Do something geometric, like imagine a point, then a line, spin the line around. Then unfold it into a triangle, then make it into a pyramid. Then go back into a triangle and start to make your own fractals.
Of course the obvious choice is to learn a language! Try Rosetta Stone. It is really nice. You just have to do it and you will learn. It is not like some computer programs where you have to put the bits and pieces together. It has a comprehensive approach.
I think writing is always exercise for the brain, especially, if you try to stretch your verbal limits.
Learning a rap, like 2pak or Emimem or whoever. Once you have figured out the words, repeating it back right on is a challenge for most people. And that means more than the chorus, you know?
Aural, tactile, olifactory
For these you can switch imagining the sound for the touch for the smell for the taste, you know?
Hear different sounds from nature, like the ocean or a waterfall or a fire burning. Bring the different sounds together. Imagine they are in different places, sort of like you are surround sound.
Imagine new sounds that don't exist or that you have never heard. What would a catepillar walking sound like? Hear it in your head. Etc...