My favorite go to, one I've used twice in the same campaign and no one was the wiser, is to throw some ridiculous fight at the party out of nowhere, let them sweat it out for a round or two, and start dropping hints it isn't what it seems.

I had them stumble across a black dragon in a cave as a lvl 1 party once. After scaring the shit out of them, for a round or two, someone "finally noticed" that the wings seemed to be made of tar covered cloth. Druid did a nature check and realized that's not what a black dragon roar sounds like at all. Literally 5 kobolds in a dragon coat.

One time, I thought we had canceled but everyone pinged me about why I wasn't logged in to roll20 yet (got my weeks mixed up). Luckily one other person did too, so I told the party I was going to puppet their character so they would level up too. I had that character betray the party by leading them to a trap. They defeated the player character (I used their actual character sheet to fight the party), for them to discover it was a doppelganger, and the trap was the diopleganger's lair. they solved through a bunch of traps and random creatures from the diopleganger's managerie of tortured -to-the-point-of-insanity minor monsters until they found the actual player character that (as they discovered) had been kidnapped the night before.

One other time l, over lockdowns, I had a friend miss a few months of sessions due to some serious and very depressing circumstances. He still wanted to continue once life had calmed down. We were doing an Avernus campaign, and I had been NPCing his character, but I told him to fast forward to his character to the current party level (about 6 levels) and not tell anyone he was going to rejoin the play sessions or log into roll20 until I gave him the go ahead. About 15 minutes in, the party is sailing down the river Styx when they see a damaged flying fortress crash landing, streaking by overhead. They hear a hellish scream and see a buck naked tiefling jumping out of the ship directly for their raft. At this point my friend logs into discord and yells "I WANT MY SHIT BACK YOU IMPOSTER BASTARD!". combat began immediately whereupon he fought himself and regained all the loot the imposter had been carrying. The party had a hell of a good time that night, and he never did explain (in character) what hell actually happened to him.