Back in my childhood, I remember sometimes getting tagged along for walks, and then wondering what to actually do.
Even bringing a stick along helped, I would think of something to do with it.
Nowadays, e.g. if I don't have (enough of a) normal workout planned, I'm thinking of ways to utilize a normal walk.
From simply walking more consciously, i.e. re-assessing, tuning, or even potentially re-learning how to walk: Where does each foot make contact? What angles? How to I roll off? Do I overpronate? Supinate? Lack stability? Are my shoes doing me good?
... to bringing 2 racquets long and going through shadow swings *and* movements (aka matching footwork).
Practicing to slide, especially on the less skilled leg.
Maybe bring 2-3 balls to improve juggling while moving.
Chance for us to go back to being kids and explore what's around us? Balance, jump, climb, run?
Similar to optional downtime, we don't *always* have to think and work, but it would seem that whenever we want to, we can come up with something...