In keeping with this season's theme of going "back to the beginning", in the shot right before the opening credits Buffy greets Rona by saying "Welcome to the Hellmouth", which is the title of the first show of the series.
Xander is shown sleeping on the couch at Buffy's house, completing the Scooby Gang's migration to the Summers residence.
The barrier Willow uses to stop the Turok-Han from entering Buffy's house is the same barrier she used in Spiral (2001).
When Andrew says he's bored, "Episode One bored," he's referring to 'Star Wars: The Phantom Menace" (1999), considered by most Star Wars fans, of which Andrew is one, to be the worst entry in the series.
The fact that Buffy might have brought back something evil from the grave with her was first introduced in After Life (2001), when she had to fight a demon created in thaumogenesis from the resurrection spell.