An off season is a period of time in which there is no upcoming competitions or events and training usually transitions into a more generalised state.
Many people assume powerlifting is just singles and the big three, which couldn't be further from the truth. In fact around 80% of all training is accessory or muscle building related training.
Personally in my off season I work on cardio, specifically my mile run, I work on my martial arts and general building muscle and strength.
You can see where I'm going with this basically do bodybuilding and side quests. Whilst its not perfectly optimal I make sure all my grounds are covered and that powerlifting comes first.
So where does that leave you?
No matter the sport you need to identify your hierarchy of specificity. The bottom being most general and the top being the sport itself, for example the bottom of a powerlifters hierarchy would be cardio, muscle building, and a large range of accessories. A Boxers would be strength training, muscle building and other activities that build the boxers ability to perform in the ring without actually fighting. Strength and Conditioning etc.
Your off season is the base of your hierarchy. The closer to competition the more specific you get. Enjoy your off season, reach the goals you don't normally get to do, gain weight, lose weight, train a new movements. work on mobility, cardio, get bigger, faster whatever you need to do.