In the '60s, Tom Kennedy played "You Don't Say!" once a year while brother Jack Narz hosted.
If we're counting the Clark/Cullen "Pyramid" thing, then let's count Vicki Lawrence playing the nighttime version of "Win, Lose or Draw" while she was still hosting the daytime version.
Also, in addition to the '70s ABC version, Allen Ludden played a few weeks of the daytime "Password" in the '60s.
Brendan