Take a Nap
Taking a brief power nap is honestly one of the best ways to boost brainpower and productivity. If you can manage a quick 10-30 minute nap before you need to do something important, you'll likely be more focused and alert when you do have to perform. Don't nap too long though, the benefits of a power nap tend to fall off the longer you snooze.
Learn to Spell Things Backwards
This falls under the same category of "reading upside down." This practice forces your brain to view recognizable stimuli differently. First, try to learn the alphabet backward, then try to write sentences backward.