Person A of your OTP gets a lobster in their pants during a fishing trip (Person B is completely baffled as to how this happens) and Person B attempts to get it out while A screams and hops about. B is laughing so hard they can’t breathe. -- Gallifreya
Frisk's therapist had recommended fishing as an occupation that might get them to talk a little more. And since it was do as the therapist said or suffer Frisk to wind up back in foster care, the entire extended family went camping.
Mama Toriel, Dunkle Sans, Uncle Papyrus, Uncle King Asgore (who slept alone in his own tent], and the Aunts Alphys and Undyne. And, for some reason known only to himself, Mettaton.
He had solar panels on his tent for recharging and spent most of his time gathering raw footage for his proposed show, In the Wild With Mettaton. It was looking to be a documentary/reality show with a side of romance.
But it was really an excuse to try and get cosy with Papyrus.
"Papyrus, darling, can you show me how to get this super wriggly worm onto a hook?"
"Papyrus, sweetie... show me how to cast like that?"
"Oh! I dropped my solar parasol in the water."
Mettaton overheated whenever Papyrus got his shirt wet. And giggled a lot. And put himself in the most ridiculous poses.
And Papyrus, blithe as ever, completely missed every last signal Mettaton threw at him.
"AH!" Frisk had a bite. The entire camp crowded in to help, though Alphys insisted that Undyne refrain from using her spears on the animal. And once it was reeled in...
"How could you catch a lobster in a river that is miles from the ocean?" wondered Toriel.
"Maybe it got lost," suggested Sans. Who was inexplicably wringing wet and hiding a snorkel behind his back.
"L-lost," muttered Alphys. "Sure."
"What shall we do with your prize, my child?" asked Toriel. "Try to use your words, please?"
The assembled monsters held their bated breath.
"Heal," said Frisk. "Mercy." And, after a moment's thought, "Pet."
There was great jubilation, while the new pet was introduced to a suspiciously convenient saltwater travel tank and fed with some equally suspiciously convenient lobster food.
Undyne ticked the checklist item that said, "Frisk talks during trip."
"Okay, kiddo," said Sans. "Let's try and catch some small fry for your pet to eat. It is it gonna get a name?"
The campfire was the best part. Toriel snuggled with Sans. Undyne snuggled with Alphys. And Mettaton kept scootching closer to Papyrus, who kept edging a little bit away.
The fire was warm, but not dangerous. The stars were spectacular.
Sans got up to check on Snappy the Lobster, and then boxed Papyrus in on the chosen log he was using as a seat. Which allowed Mettaton to finally close in and attempt to snuggle.
Which is what would have happened if Sans hadn't put Snappy the Lobster into one of Mettaton's arm holes.
We will leave the resultant scene of destruction to your imagination.