Hmm, I'm thinking of my own to add, because they feel so far away even though they're comparatively not:
1) A bunch of us on the sorority hall last-minute deciding to go to a pajama-themed mixer with another sorority and then wandering into the Jimmy Johns downtown in our pajamas to eat there on a Tuesday night.
2) Formal my junior year where one sister decided to wear all the sashes at once for all the superlative winners who didn't get theirs-she was wearing multiple white satin sashes at once that said all sorts of things like "candlelight here we come", "most likely to be found at a fraternity house", "pledge class mom", etc. and running around screaming "I'm a winner!"
3) On a more serious note, I don't think I ever bonded as much with my sisters as when the younger biological sister of a senior chapter sister suddenly got sick and passed away while she was participating in recruitment. It was absolutely heartbreaking, but the way we honored her, supported each other and loved on the chapter sister truly showed me the meaning of our open motto, We Live For Each Other.
And of course there was the time my mom surprised me during ritual, but that one is during ritual...still my favorite happy memory from ADPi
I think it was Sciencewoman who guessed correctly that she was coming to surprise me when I posted about it back in the day.