I am an occupational therapist and a practitioner of something called brain gym. I am also doing futher study on other modalities to enhance brain function. Form what I know and what I've observed in working with children and adults, the brain is plastic(like you said) and though it slows down as we grow, the area in the brain called the hippocampus, which has to do with acquiring new information and memory, is active until we die. Also, doing puzzles and such may feel like you're working out your brain, but the best, quickest way to provide alerting input to your brain is through movement. If you think about how the brain knows what is going on around you, is that it uses the senses and without input from the senses, the brain in essence goes to sleep. (when I say senses, I mean the ones we typically think of i.e. hearing, seeing, touch, smell, taste, as well as other internal senses such as proprioception-knowing where your body is in space, and vestibular input-key sense in muscle tone, head position--this is very simplified by the way) There are other factors to consider from a sensory integration perspective if you for one do not enjoy the sensation of movement or tend to have low muscle tone which makes it more challenging or (feel like) it takes more "effort" to move. So in my experience and opinion, the best thing to do to "wake up" your brain immediately, is to move your body--bike, run, walk, jump on a trampoline, whatever. There's a good book by a woman named Carla Hannaford( a neurobiologist) called "Smart Moves: why learning is not all in your head" and it talks more in detail about how our neurology is hooked up to the body. It's really good and maybe helpful in answering your questions.

Glad I gave up television long ago!