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 US TV show The Midnight Special (1972) in 1975. Moon's set featured a second, unused bass drum filled with water and live goldfish.
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 the 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).
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.