The off-season is the time to give yourself a well deserved break from constant hard-core training. It's a time to regenerate and focus on other things. This will be beneficial when the training season starts again.
Here are some good ideas about what you can do during the off-season:
Decrease the volume. There's plenty of time to hit the long trail. Keep the distances down. When it's time to start increasing the mileage you'll be rested and ready to take on the task.
Improve technique. We can all improve our technique, whether it's running, cycling and most especially swimming. If you need to take lessons, now is the time. Triathletes should focus on the weakest sport. You'll reap many benefits.
Strength train. It seems obvious that strength training will help you be stronger, but studies consistently show that it will help you improve running, cycling and swimming efficiency. Chances are you know what part of your body is your weak link, work on it now so that you won't have one.
Try something new. If you've been talking about yoga for a few years, sign up for classes, or pilates or zumba. Moving your body in different way will help stave off over use injuries. It's always good to use your muscles in a different way.
Eat well. Many endurance athletes use the off season time to eat junk they wouldn't dream of eating while they are training. Keep your nutrition up and a cookie now and then won't hurt.
If you use the off-season well you'll begin your training season stronger and rejuvenated. And that will make for great training and great races. Win-win.