If you have been participating in a certain exercise for a while now (eg. yoga, martial arts, running, archery, boxing, etc.), consider heightening your understanding of its history or inner-workings to find more fulfillment within the practice and to even improve the physical aspect of the activity itself.
Some examples of this include:
Runners - Learn the history of marathons. It's fascinating.
Dancers - Take music theory lessons. Learn rhythms and melodies.
Cyclists - Learn how to fix/tune up your bike or even build one from scratch.
Martial Arts - Learn the history of your and other disciplines.
Weight Lifters - Teach yourself about anatomy and psysiology.
Yoga - Learn the benefits of each posture, the history of yoga and the different disciplines of yoga (breathing, meditation, and so forth).
Deepening an activity with some history and theory will turn a simple exercise from a hobby into a lifestyle.
Sample Books if you are into Archery: 'Precision Archery' is a great practical advice book about everything from equipment maintenance to cross-training exercises to proper form. 'Zen Bow Zen Arrow' gets more into the mental discipline of archery, but is also great for motivation to get you out on the archery range and practicing regularly.