Garth's drumming bit in the shop? That was really Dana Carvey playing. No double there.
Charlton Heston's cameo in Wayne's World 2. Some acting classes even show that scene to students to demonstrate the difference between just reading lines and actually acting.
Garth saying that he'll kill Honey's husband but once away from her refuses to do it.
The YMCA scene in the second film. While spying on Cassandra and Bobby, Wayne, Garth and their friends, who coincidentally are dressed like the Village People, have to run away from Bobby. They end up running in to a gay bar and attempt to escape through the exit at the back of the stage. They can't, and the DJ notices them and starts playing "YMCA." They make the best of the predicament and proceed to bring the house down with their rendition of the song while, simultaneously, the crowd prevents Bobby from getting to them.