When Buffy tells Giles she is 'still going ick' from the last time he tried to recapture his youth, it is a reference to Band Candy (1998), where the adults turn into teenagers and Giles has sex with Buffy's mother.
(at around 41 mins) Willow is seen hanging a poster in her dorm for Oz's band "Dingoes Ate My Baby."
The Mok'Tagar demon heavily resembles the demon Toth, who will later appear in The Replacement (2000).