Non-Singing Voice: In the first movie, John De Luca was dubbed over by Jason Evigan, as they originally wanted him to have a more Elvis-esque voice when he sang, as you can hear. Like Zac Efron before him, though, they let him sing his parts in the sequel. Ditto for Garrett Clayton, whose singing was dubbed by country singer Spencer Lee in most of the first movie's songs the character sings in.
Recycled Script: Watch the second film, then watch the live action Fat Albert movie. They share almost identical plot layouts and beats.
Politically Correct History: There are more non-white characters in the Wet Side Story movie in secondary roles than you'd expect for an early-60s beach movie.
Throw It In!: In the first movie, Brady wasn't supposed to be part of 'Cruisin' For A Bruisin'' at all, but after the director saw Ross Lynch's dancing last-minute, they decided to rewrite the scene to have Brady in it. If you pay attention, you can hear Ross' voice during the first chorus before his first verse starts, signifying that this was a last-minute change, as if it were something in there from the start, they probably would've edited Ross' voice out of the mix for that one. It fits so well in-universe, though, that it makes you wonder why that wasn't the plan for the start.
What Could Have Been: Apparently, Tanner was originally supposed to be played by Ross before Garrett Clayton was chosen for the role, and Brady was originally supposed to be more of a background character till they added more scenes with him to the first movie to beef up his plot involvement. This lets the fact that Brady doesn't really develop much in the movie, as well as the rushed-ness of the beginning, make much more sense.