If you need a few ideas to get your creative juices flowing, consider the following.
If You're a Runner: Runners benefit from strength training, core workouts, and flexibility training, particularly through the hips. Adding a quick strength training routine to your weekly runs can help balance out any muscular imbalances. It's a good idea to also set aside time for stretching or yoga each week.
If You're a Yogi: Yoga does amazing things for flexibility, balance, coordination, and some muscular endurance, but it won't drastically improve cardiovascular endurance or muscular strength. Consider adding a circuit training workout (that combines strength training and cardio) a couple times a week.
If You're a Strength Trainer: As great as it is to hit the weights regularly, it's also important to maintain flexibility and develop cardiovascular endurance. Add a 10-minute stretch after each of your strength training workouts and on the days you're not at the gym, consider playing a sport, such as basketball, tennis, or soccer.
If You're a Cyclist: Cyclists have awesome lower body strength and endurance, as well as killer heart and lung health, but they may not be enjoying the benefits of workouts with more impact. Try adding an upper body strength training routine on the days you hit the bike, and on your rest days, incorporate hiking or rock climbing.
If You're a Swimmer: Like cycling, swimming delivers stellar cardiovascular improvements and muscular endurance, but it's not going to lead to significant gains in muscular strength or bone density. When you're not in the pool, add a boot camp class to your routine to take advantage of the benefits of low- to high-impact strength training.