Pop culture jokes for the older viewers.

The most recent (and obvious) outside reference is in "When Worlds Collide" about another planet heading toward Earth.

-Melancholia

-Another Earth

Perry's group watching Niagra Falls in the same way as Ocean's Eleven watched the water show in Vegas.

In an episode where Doof and Perry are working together because they are being hunted by a platypus hunter. They are making traps, weapons, and big glove for Doof and he uses it to crush a fruit. He then says, "Groovy." Similar scene in the movie Army of Darkness where Ash is making all sorts of weapons and a mechanical hand that he tests by crushing a metal cup and says, "Groovy."

The one with the Lake Nose monster, muttering "Nosebud" and dropping the snow globe - a Citizen Kane ref. :)

I'm trying in vain to come up with the episode, but there's one where the boys create the Ultimate Computer, who creates a Pink Ray that scans the Tri-State area. As it hits Norm, Norm goes "Get out of my brain!" ... which I'm pretty sure is a direct reference to Philip K. Dick's novel "VALIS", which has the main character in the novel be enlightened by a pink beam of light that pretty much pushes him off the cliff of insanity.

Have they referenced Walking Dead and/or Game of Thrones yet? Both their principal voice actors appeared on those shows!

The episode "mind share" had a lot "The Shawshank Redemption" references, including a okay imitation of Morgan Freemans 'Red'

IN P&F save summer - Monty's son says, "Great, they have a cave troll" Boromir says that in the Fellowship of the Ring

In Bee Day Bee Story the female villain does a scene like Galadriel when she is temped by the ring in The fellowship of the Ring

Isabella does a speech like John Belushi does in Animal House "Was it over when ...."