After the credits, while Jerry is looking in the fridge, the bully from Edge of Tomorty: Rick Die Rickpeat (2019) who was sent into Earth's upper atmosphere by Morty is on a milk container, listed as missing.
When Jerry opens the fridge in the after credits scene, you can see the passing-butter-robot that Rick built from Something Ricked This Way Comes (2014).
The QR code on the funnel Rick is wearing when he goes to the robot war works if scanned by a phone app and redirects you to the Rick and Morty store where you can buy a funnel with the same QR code on it.
Rick gliding his hand over a field of ripe grain on his way to his toilet bowl is a reference to the beginning and ending scene from Gladiator (2000).
The title is a play on the Ernest Hemingway novella 'The Old Man & The Sea.'