Not sure why but this crossed my mind earlier today.
I recall Dick Clark as a celebrity contestant on Bill Cullen's $25,000 Pyramid.
Chuck Woolery was a player on Scrabble one week during a game show host tournament.
Peter Marshall was the center square during a game show themed week on Bergeron's Hollywood Squares.
Tom Kennedy once played Password Plus while he was the actual host, and his brother Jack Narz was host for a puzzle (maybe more).
Bert Convy sometimes appeared with his wife on Tattletales whenever they used other rotating hosts.
Gene Rayburn once sat in Charles' seat during a Match Game question after Gene told Charles how tired he was and Charles had in jest made a complaint about Gene not coming down to the end of the row to see Charles' answer card.
Both Bill Cullen and Joe Garagiola served both as emcees and celebrity panelists on To Tell The Truth.
Pat Sajak and Vanna White were contestants on Wheel during April 1, 1997.
While never playing a full game, Bob Barker I know spun the big wheel for some contestants.
I'm sure there are other examples of this that are not coming to mind for me just now. I'm curious to hear of other recollections as well.