The Brain Is Like A Muscle. When It Is In Use We Feel Very Good. Understanding Is Joyous. Carl Sagan(Astronomer and author 1934-96)

If you want to enjoy life more, use your mind. Challenge yourself to contemplate difficult ideas. Read. Have intelligent discussions. Write your thoughts in a journal. Write emails or letters to your friends about what you are thinking. (Let me add share your ideas in a or several blogs). Italics are mine.
Give your brain a workout as you would give your muscles a workout at the gym. 

Discovered this site to make mindmaps or concept maps for revision purposes. It's called spicynodes.
I tried one on cells and am posting the link here for whoever wants to have a go at it.

http://www.spicynodes.org/a/7516d4dd400e57862914b9b79e8f2f65