Here's another wiki (website resource we can all contribute to) that took me off my trail just long enough to get refreshed:
Simply put, all one needs to grow his/her brain is to do unique, random, different, and ridiculous things as often as possible. One hour of increased brain activity via thinking a lot or experiencing new stimuli can make you smarter, more energetic, more creative, and more sociable. Here are some of the endless numbers of activities that can stimulate your brain. The one key ingredient is to have an open mind. – How to Exercise an Open Mind
One of my favorites is #4.
It reminds me of a writing exercise (also great for actors), that asks you to memorize as many details in a given location as you can (for example, while you're waiting at the bank or post office), including colors, lighting, sounds, shapes, textures, etc., not just naming “chair,” “table,” “door,” which are too simple and nondescript.
This helps to build variety in your sensory memory which you'll use each time you need to build a character or event. Just like painters who add and combine more colors on their palette, you'll be adding more choices.