The set used for Pete's living room is the same set used for the Stephen's living room on Bewitched (1964). The exterior set for the Stephen's house on Bewitched (1964) sits next to the exterior used for Gidget's house so it stands to reason that Pete lived next door to Gidget.
Both Beverly Washburn and Bonnie Franklin make one more appearance in this series, but play different characters from the ones they play here. Nor do they appear again in the same episode.
Bonnie Franklin would go on to star in her own TV sitcom in the 70s, playing that of Ann Romano in the long-running series One Day at a Time (1975), a role for which she is perhaps best remembered.
When Gidget says, 'In the immortal words of what's his name, the die is cast', she is alluding to Julius Caesar who crossed the Rubicon River into Italy on 10 January 49 B.C.E. with his army, pushing the Roman Empire into the throes of civil war. He supposedly said, 'alea iacta est', i.e. 'the die is cast', after completing the crossing.
Barbara Hershey appears two more times in this series, reprising her role of Ellen both times, i.e. in Love and the Single Gidget (1966) (although mistakenly listed in the credits as Karen) and Ask Helpful Hannah (1966). She, of course, went on to greater stardom later in her career, winning multiple awards and nominations: The winner of an Emmy and a Golden Globe for A Killing in a Small Town (1990), consecutively winning Best Actress awards at the Cannes Film Festival, (an unprecedented feat) for Shy People (1987) and A World Apart (1988), respectively; winning a Gemini Award for Anne of Green Gables: A New Beginning (2008) on PBS. She was nominated for an Academy Award for her work in The Portrait of a Lady (1996).