stretching vs exercising

stretching and exercising both are nouns.

Word	Noun	Adjective	Verb	Adverb
stretching	Yes	No	No	No
exercising	Yes	No	No	No

As nouns, exercising is a hypernym of stretching; that is, exercising is a word with a broader meaning than stretching:

stretching: exercise designed to extend the limbs and muscles to their full extent
exercising: the activity of exerting your muscles in various ways to keep fit

Other hypernyms of stretching include exercise, physical exercise, physical exertion, workout.

stretching (noun)	exercising (noun)
exercise designed to extend the limbs and muscles to their full extent	the activity of exerting your muscles in various ways to keep fit
act of expanding by lengthening or widening