Do with a gerund (like stretching ), though grammatical, is not very idiomatic. And with a quantifier it is completely idiomatic.

So do stretching sounds odd ;but do some stretching (or a bit of stretching or a lot of stretching) is fine. Is there a way to do it please?

Now I think of it, the same is true for the nominal stretches : do some stretches (or do a few stretches ) is much more natural than do stretches.

Difference between stretches and stretching? What’s the difference of meaning?