i have found my new favorite type of greek genitive, courtesy of m.l. west. it is in hesiod, theogony 5-6.
καί τε λοεσσάμεναι τέρενα χρόα Περμησσοῖο (5)
ἠ’ Ἵππου κρήνης ἠ’ Ὀλμειοῦ ζαθέοιο
you know what that's called? prof. west tells us: the 'genitive of water in (from)which one washes'. two further examples he cites are il.5.6 and 6.508.
UPDATE: by the way, i'm having a hard time getting the greek font to work. oops.