The bass that Peter plays "Tom Sawyer" on, during his first jam with Sydney, is a Fender Geddy Lee Jazz Bass from Fender's Artist Series of electric basses, and is a near-perfect replica of the instrument that Geddy Lee usually plays onstage with Rush. Geddy can be seen playing the original instrument during the Rush show, later in the movie.
The drum set in Sydney's "man cave" is a replica of Keith Moon's (The Who) infamous Vistalite "Goldfish" set, as seen on the U.S. TV show The Midnight Special (1972) in 1975. Moon's set featured a second, unused bass drum filled with water and live goldfish.
Much of the cast starred in or made a cameo on the TV show Parks and Recreation (2009-2015), including: Paul Rudd, Rashida Jones, Aziz Ansari, Nick Kroll, Rob Huebel and JK Simmons. Andy Samberg and Paul Rudd also made appearances.
When Peter is in Sydney's man cave for the first time, he sees a photo of Sydney with friends in high school. The school name in the picture is Westlake High. In real life, Jason Segel went to a middle/high school named Harvard-Westlake School.
The cameo of Matt Walsh as the impatient golfer comes from his original character in the famous "Ass Pennies" skit performed under the comedy group The Upright Citizens Brigade. In addition to being the same actor, he wears the exact outfit from the original 1990 skit in Upright Citizens Brigade: Power Marketing (1998).
Sydney's dog is named Anwar after the former president of Egypt, Anwar Sadat who was murdered in 1981. Following the movies' release, one of Sadat's daughters sued the producers because in Egyptian culture, a human compared with a dog is a serious insult. The lawyer representing Sadat's daughter, requested a formal apology from the U.S Embassy in Egypt, which never came.
Paul Rudd (Peter), Jon Favreau (Barry), J.K. Simmons (Peter's dad) and Lou Ferrigno (who plays himself), all have a connection to Marvel Comic's movies. Paul Rudd played Scott Lang/Ant-Man in Ant-Man (2015). Jon Favreau directed Iron Man (2008) and Iron Man 2 (2010) and starred as Happy Hogan in all three Iron Man movies (2008-2013), and he played Foggy Nelson in Daredevil (2003). J.K. Simmons starred in all three Spider-Man movies (2002-2007) as editor Mr. Jameson. Lou Ferrigno provided his voice for the Hulk in the movies and played the Hulk in the TV series, The Incredible Hulk (1978-1982).
Sydney mentions to Peter that he tries to go to the blood bank often because he has a rare blood type, AB negative. While AB is indeed rare, it's also the least useful type for a blood bank to stock. Type AB can only be given to others who have blood type AB, but such people can accept ANY blood type, hence Type AB being called the "universal receiver". This is the opposite of Type O, which is highly valuable to a blood bank because it can be given to anyone, hence it being called the "universal donor".
Rob Huebel (Tevin) says that he got an e-mail from a buddy in his fantasy football league. Huebel is often in the hit tv show "The League" on FX, which is about a group of friends and their fantasy football league.
During a dinner scene following dialogue from Jane Curtin, who is one of the original cast members of Saturday Night Live, Peter and his second "mandate" discuss their favorite "old Saturday Night Live" skits.
Throughout the movie, we see several instances of Apple's iPhone being used (mainly by Peter), along with its staple "Marimba" ringtone. Later on in the movie, we can hear Matt Costa's "Mr. Pitiful" playing after Peter argues with Zooey about not being sure about their marriage, which was used by Apple later on in their first iPhone 3GS advertisement.
Paul Rudd had a small role in the film Jason Segel was the star of the previous year, Forgetting Sarah Marshall (2008). Rudd played a pothead, who worked at the surfboard rental hut, who rented a surfboard to Segel's character and who also helped on the beach when he sustained an injury from coral.
The trivia item below may give away important plot points.
Peter asks for his Lost (2004-2010) DVDs back. Paul Rudd is a huge fan of the show. This is also a topic in another movie he starred in, This is 40, when one of the daughters watches all the episodes of Lost in a condensed amount of time.