The wording for a surprise party invitation should implicitly state that the party needs to be a surprise, in addition to information concerning the location, time, date and RSVP contact numbers. A fun phrase such as "The streamers are hung, the lights are all low, the guests are all hidden, so let the confetti flow" lets everyone know the party is unexpected.
Another poem to tell guests the party is a surprise is "A birthday is near, but let's keep it hush hush. Let's celebrate with a party and make the birthday girl (or boy) blush." The invitation should contain any information about a theme, the address for the party and the date that the guests must RSVP by.