Elopement: What the family thinks Alice and Sam are doing. What Alice's cousin (offscreen) is actually doing (and Alice has been asked to be a witness).
Exactly What It Says on the Tin: Alice asking for the weekend off to accompany Sam to a private wedding ceremony for one of Alice's cousins is exactly what she says it is despite the family thinking it is something else.
Not What It Looks Like: To the family, as they're forever jumping to conclusions that it is Sam and Alice that are getting married ... even to the point where they hastily plan a surprise wedding party on the night Sam took Alice to a prestigious bowling tournament.
They Do: Averted in the series, as the series was cancelled before a wedding-day script could be completed. Had the series been renewed for a sixth season, such an episode would likely have been a fall 1974 episode, perhaps a season premiere. (Incidentally, "They Did" sometime between the final episode and "The Brady Girls Get Married" in 1981, and were still married as of The Bradys, although Sam was not seen.)
Will They or Won't They?: Will Sam ever ask Alice to marry him? Will they elope off-screen over the weekend? The latter point no; the former ... as Alice reveals in the tag scene ... a resounding yes!