In most cases, a single session won't be enough in order to achieve any main goal that the player characters had in mind at the time. It could even take several sessions just to go through a single dungeon, or travel to a major area. With the sessions on hold, it's easy enough to pick up right off next time.

But sometimes, either by the DM's thinking that the session would take longer due to the challenge of the goal, or through the PC's suddenly shifting gears on what they want to do, the main meat of the session is suddenly eaten, and the PC's, having done everything the DM has made for this moment, are now looking around thinking "Now what"? Tell a story about a time where your group ended up having a good amount of time to spare in a session after accomplishing a certain goal.

I felt that way when I beat Little Nightmares the day after I got it.

I found that Charm spells can quickly end a session. Twice, I have had a Charm Person spell end an encounter or module early.

First time was when the party was asked to retrieve an over zealous noble who wished to be an adventurer himself. The plan was after rescuing him from the first combat he got himself into, he was supposed to lead the party on a merry chase scene into another fight. *Charm Person* But why would he run away from his best friends?

Second time was when the party was beset by a Pirate Ship. After disabling the crew with Hypnotic Pattern, they charmed the Captain, leaving only the near dead mage to deal with. Who then proceeded to argue with the now charmed Captain. Yeah, in that case DM fiat stated that the Captain's patron did not like having his pawn suborned, pushing up the timetable on the second fight of the day.

But if you do the Charm Person all the time, GM will start to put enemies with amulets of charm protection. Or the ememies that cast charm person on you, to make you taste your own medicine.

as long as they don't send witches with sleep hexes. turning helpless in a mele is only fun when the enemy does it, when it happens to your surrounded tank, it's pretty darn close to TPK.

against charm spell, you come up with something far to alien to be charmed by a human (or whatever the player is playing) and they happen to come across these thing everyfreakinwhere.
that or some form of protection against charm spell