In the movie Hook, George Lucas and Carrie Fischer made a cameo as a couple necking on the bridge when Tink takes Robin Williams to Neverland.

In Mystery Men, Michael Bay shows up in the villain meeting as the leader of the Frat Boys.

In Austin Powers 3, Steven Spielberg shows up as himself in a film shoot with a film type of gag. (Checked his wiki and he cameoed in other films as well, Gremlins 1 & 2, although I've never found him in all my viewings)

Harold Ramis as Egon in the Ghostbusters 1 & 2.

Rob Reiner is the antagonist TV executive in EdTV. I know he shows up in a lot of 90s romcoms. Most he directed himself.

My recall for this trivia seems to be more 90s. lol