Hit the Track
There's no time like the present to finally nab that 800-meter PR. Hit your local indoor track to see if there are any all-comers meets coming up, then have fun racing the shorter stuff. (To prep, add some speedwork to your running workouts and always properly stretch and warm up beforehand to avoid injury).
Yes, a 3,200-meter race may in some ways feel tougher than a half marathon, but just imagine how fresh your legs will feel when you don't have to swim and bike beforehand.
Take a Spin
Or, if you're seeking something more social, work an indoor cycling class into your routine, like Spin90, where you ride your own bike on a trainer under the watchful eye of a coach. (Talk to the folks at a bike or tri shop to see if there's one nearby.)
Try something new! Whether it's TRX, CrossFit, that new Zumba class or Pilates, add a little variety to your workouts. During the offseason, dedicate a day (or two) to something out of the box or that one thing you've always wanted to try but haven't had the time. Or, hit the slopes, go snowshoeing, or charge up the hills while sledding with the kids and call it a workout.
Because, above all, you want to give yourself a break from the intensity so you can steer clear from injury and burnout and be able to enter your next racing season super-motivated--and super-fit.
Search for your next triathlon.