In 1998 we were on the Sun Princess when they filmed some episodes for Love Boat, The Next Wave. Lots of never-heard-from again actors and actresses, but the late Robert Urich was great. I almost knocked him over one morning in the breakfast buffet when we rounded a corner at the same time.
And then there was Marty Allen on the Grand Princess in 2001. He would hang around in public areas quite willing to chat it up with anyone who wanted to talk.
Not exactly a Hollywood star, but one of the bands I played with on Princess alternated sets with Les Brown and his Band of Renown--that was fun--and scary! It's not ofter you get a chance to jam with musicians of that caliber. And the bull sessions were fun too; nobody can sling it like musicians.