Kind of depends how much you want/need a break from SBR training and/or structured training.

Personally I find I need a period each year where there is no plan and I just go with whatever I feel like doing each day for a few weeks up to a few months. That could include doing nothing but more often than not I still stay pretty active and fit but just have fun. E.g. Might do some non-SBR sport like tennis, golf, kayaking, SUP, weights. Do some social rides without fretting about how low the pace is. Etc. I lose some specificity and gain a few pounds, but I come back mentally and physically fresher and ready to tackle serious training again.

If you don't need as much of a break then general advice is to work on your weaknesses. E.g. If swimming is a weakness then now would be a great time to put running and cycling on the back burner and swim 5 times a week. If no particular weaknesses then building a run base is probably what I'd do, as that's the sport where IME the improvements take longest. Build up run volume with lots of easy running. And also the best sport for losing weight.

Personally I let desire be my guide during those periods. In other words, after I get up I essentially do whatever sounds fun or interesting to me at that moment. It could be a short ride, easy run, walking the dog, paddle board, etc. Heck, sometimes it’s go back to sleep or sit on the couch with the dog and a coffee. After a few weeks that’s when I’ll start thinking about workouts again.