to stretch (If you stretch yourself, you need to use a self reference in German "sich+Acc strecken". In a context of sports, "sich dehnen" is the more common term)
to unbend (For arms and legs.)
Opposite (closest): beugen
to dilute (For substances only, including drugs. The core theme is that you make it "last longer" because it's "more". So the focus is not primarily on making it "thinner".)
def 3. typo? do you mean dilute?
Yup, fixed it, danke :)!