“Wearing just leggings is no good because skin-tight clothing is considered no different from being naked.”
That is absurd. The Gemara says that something transparent is also assur, but skin-tight is hardly the same thing as transparent. See the teshuva from R’Moshe cited by Sam above.
Theoretically all spandex would be in keeping with the halacha. However frum Jewish women the world over have a broader sense of tznius than the bare minimum and keep a higher standard. The standard a woman must keep is dependent on her community and her personal sense of tznius (l’chumra only).