-- Helena brainwashed Luke to be her island lover and they've been luxuriating on a beach somewhere and plotting some sort of attack on the people of Port Charles.
-- Helena kept Luke locked up in a cage where her team of scientists created a genetic clone that combined Spencer and Cassadine DNA into an impeccably manicured -- but prone to alcoholism -- Cassadine heir.
-- They've secretly been running "The Amazing Race." They finished third.
OK, that's just to get you started -- more suggestions in the comments, please! The possibilities may well be endless.