This summer, my sister and I threw my parents a small, immediate family, surprise party. Due to the distance that people would be traveling and busy summer schedules, we sent out Save The Dates a few months prior. Due to the surprise element we had to track down photos of them as children without ruining the surprise. For the Save the Date we used current photos with the intention that the invitation would have photos of them as children when we could artfully procure them.
As an extra element to add texture and to bring attention to the Surprise, we tied "shhh....its a surprise" onto the invitations with ribbons in different shades of orange.