The offseason can be tricky for athletes. Some love it, some hate it, some need it, and some stay in it too long.
I view the offseason as a time to refresh your mind and your body. It's your time to be a normal healthy person for awhile. It allows you to get organized, catch up on items that you've neglected the past 10 months, spend extra time with your family, etc.
The key point, is to be a normal "healthy" person. Don't treat the offseason as a time to drop your guard and let yourself go. If you've created a balanced lifestyle, the biggest difference is just the extra time you'll have on your hands. You might choose to indulge a bit more than normal, but those that go crazy end up regretting that decision come January or February. I know because I've been there!
Once you've broken your routine, put on excessive weight, lose range of motion and strength...your ability to get back to your routine plummets. You'll want to get going again, but to get started becomes a real challenge. Being a triathlete starts to become black and white...you're either living like a roadie for Led Zeppelin or you're living like a triathlon Monk. There is no balance and before you know it, your offseason has lasted a month longer than expected...and it's going to take an extra month to get back to where your body feels ok training again.
Some may like the big swings of either being on or off, but in my view, the best sustainable option is to be balanced across the year...all things in moderation.
What should an offseason look like?
We are all training and participating in triathlon for our own reasons. Because of that, I don't think there are any set rules for an offseason. It can be as short or long as you'd like. I personally like 3-6 weeks. During the offseason, my only rule is to keep moving. How much? Well, The American College of Sports Medicine recommends 30-60 min of aerobic exercise 5 days per week. Add in a bit of strength work and this is very manageable and would be considered a recovery week by most triathlon standards. And, since there is no rush to get back into race shape, there are a lot of non triathlon activities you can do...hiking, skiing, snowshoeing, resistance training, surfing, climbing etc.
Embrace the offseason, but don't consider it a time to live in a way that is not congruent to your health and productivity.