This is the first time urination is depicted in The Simpsons. An alien pees in the fountain before a security guard shows to stop him on which Homer was telling Bart about how guards are.
Edna Krabappel can be seen inside one of the tubes in the probe room of the spaceship.
Inside the ship, there might appear a picture of Kang and Kodos in a reference to the All your base are belong to us meme. Also, both aliens are dressed exactly like the game character who said the phrase.