Lemme see...
* Ralph Edwards appeared on Bob Barker's first
Truth or Consequences to essentially pass the hosting torch on Jack Bailey's behalf. If I'm not mistaken, he also appeared at least once on the 1980s revival.
* Bill Cullen appeared on the 1980s
$25,000 Pyramid multiple times as a guest.
* Pat Sajak stopped by during Bob Goen's first daytime
Wheel of Fortune. He also stopped by the set during an episode of
The Pat Sajak Show, a very fun segment which also saw Goen naming
Beat the I.R.S. as one of his past failures.
-Peter Marshall returning to Bergeron's Squares and hosting a round.
Not a round, but it was close - Peter did the player interviews and front game, which ended during Round 2.
Wikipedia says that Bob Eubanks guest-hosted a couple of episodes of Carnie Wilson's version of The Newlywed Game -- one so that Wilson could play as a contestant with her husband, the other with Monty Hall, Peter Marshall, Wink Martindale and their respective wives as contestants.
Can confirm, and I recommend the latter episode because it's
incredibly fun.