COMPLETE SYSTEM
The effect is simple. You throw a Balloon to the audience. You take the entire audience on an imaginary vacation. You collect 5-6 pieces of information about your trip (you can change it to anything you like). The last person receiving the balloon is invited to join you on stage. You explain that you need a voucher to go on this vacation and ask him/her to give it to you.
Of course, he/she will have no idea where the voucher is until you explain that they had it all the time and it is inside the balloon! The spectator confirms he can hear and see it inside the balloon.