Another option is to say "chag sameach," which is not specific to Passover but is the Hebrew equivalent of saying "happy holidays." To make the phrase Passover specific, you just add "Pesach." ...

“Happy Passover” is an appropriate sentiment. You could also say “chag sameach,” which means happy holiday in Hebrew, or chag kasher v’sameach, which means have a happy and kosher Passove ...