When Buffy snarks at Angel, "I was really jonesing for another heartbreaking sewer talk," she refers to events in The Prom (1999), where Angel breaks up with her while they are patrolling Sunnydale sewers.
This is the second part of a Buffy the Vampire Slayer (1997) / Angel (1999) crossover. The first part is Pangs (1999).
Buffy and Angel's kiss on the beach was foreshadowed in the dream sequence at the beginning of Anne (1998).
The outfit Buffy wore for the majority of this episode (white sweater and gray slacks) was later donned by Cordelia in First Impressions (2000) and worn by Buffy once again in The Gift (2001).
This is the second time in the series Angel gives up a chance at happiness. In In the Dark (1999), he relinquishes the ability to walk in the sun by destroying the Gem of Amara; it is also the third episode of the Buffyverse that Angel has protection for the sunlight (Amends (1998) and In the Dark (1999)).