I see it in two ways: either you divide their free time with holidays and actual training or start with progressive training of sorts. Let me explain the latter, since I think that one is a bit more difficult to utilize. Basically, you start with the first day after the season ends by seeing what exactly the player will be up to. Assuming said player is free, you just suggest to them to go to the gym for a little bit, lift some light weights, just minor stuff. You do not want them to cool down but you cannot afford a possible injury so you will start with that.

Then you progress accordingly throughout the break so that they get some rest but are kept from getting a heavy break since that switch might mess them up on the way back. That said, if people insist on the first option of taking a break and then coming back to work extra hard, I would suggest to still have progressive training just skipping the lighter stuff.