Originated as a book called "Doug Got a New Pair of Shoes".
In the episode where Doug and Skeeter form a garage band, they wear oversized suits to a gig at a birthday party, a homage to the Big Suit worn by David Byrne of the Talking Heads in Stop Making Sense.
The origin of the character Patti Mayonnaise is the combination of the names of two girls Jim Jinkins had a crush on in school.
Judy Funnie's name may come from the poem "Shakespeare's Sister" written by Virginia Woolf. In the poem, Woolf states that if Shakespeare would have had a sister, her name would have been Judith. Furthermore, Shakespeare himself had a daughter named Judith. Since Judy Funnie is obsessed with Shakespearean plays, it is possible that this is why her name is Judy.
Doug moves to 21 Jumbo Street, the street is named after the production company, Jumbo Pictures Inc.
In the pilot episode, Doug originally wrote in a "diary", instead of a "journal". In later episodes Doug would get annoyed whenever another character referred to his journal as a diary.
Doug's favorite band, "The Beets", is an obvious reference to The Beatles, most notably the lead singer (who bears a striking resemblance to John Lennon) and the drummer (who bears a similar resemblance to Ringo Starr)
Two obvious Richmond refrences (where creator Jim Jinkins was born) include: Moody School (Moody Middle School in Henrico County) and Four Leaf Clover Mall (Cloverleaf Mall in Richmond, Virginia).
In the episode where Skeeter moves into his basement, and all subsequent episodes, one of the posters on his walls is the Jumbo Pictures logo.
Doug's vice principal Mr. Bone is based on Don Knotts.
The name of Doug's unseen principal, Mr. Buttsavage, was the name of Jim Jinkins' elementary school principal.
The number 47 is referenced numerous times. For example, it is Doug's locker number, it's used on many street signs, and the liver and onions meal is meal #47 on the restaurant's menu.
The character of Skunky Beaumont is mentioned frequently in the old Nickelodeon episodes but never actually appears. He DOES appear in the later Disney-produced episodes, however.
When Doug returns from his grandmother's house, the bus line he returns on is called Beagle, an obvious parody of Greyhound (both are dog breeds.)
Doug is left-handed.
The movie character Smash Adams is based on James Bond.
A poster above Doug's bed shown prominently in many episodes features a sinking ship with the phrase "LOOSE LIPS..." emblazoned above the ship. This is actually a famous World War II-era propaganda poster, depicting a sinking ship with the words "LOOSE LIPS SINK SHIPS" discouraging civilians from carelessly talking about friends or relatives fighting abroad, as supporters of the enemy could overhear conversations and relay any useful information from the conversation, such as troop position and formation, to the enemy's advantage.
Skeeter's address is 1492 Ocean Blue. This is a play on the old "In 1492 Columbus sailed the ocean blue" rhyme.
The Funnie's neighbors are the Dinks. Their name stems from the acronym D.I.N.K. which stands for "double income, no kids". This is a slang term that refers to a childless couple where both people work and are, therefore, more affluent than families with dependents.
Man-O-Steel Man is an obvious reference to the real comic book hero, Superman.
Sometimes, Doug would write up comic books of himself when he "daydreams". One of them is Quail Man, though not a reference to any pre-existing comic book hero, but Quail Man wears his underwear over his pants, poking fun at most heroes in comic books that have an "underwear" shape design on their costumes.
There are several references to hip hop artist M.C. Hammer in the episode "Doug Can't Dance." In his imagination, Doug dances like MC Hammer while wearing parachute pants. Also, Roger dresses like a hammer for the school dance and shouts "it's hammer time."
In one episode, Doug wins a video game console called "Super Pretendo" an obvious reference to the Super Nintendo, with a game called "Space Monks" a reference to Star Fox.
Doug and the Little Liar marks the first appearance of Loretta. It's also the only segment of the show (as far as the Nickelodeon shows go) in which she is ever referred to by name.
The character of Skunky Beaumont is mentioned in many of the episodes, but never seen, as is Doug's principal, Mr. Buttsavage.