Serve the cake and ice cream before the  pizza ( or what ever you're serving.) have your guest enter  through the back door . Write out the invitations backwards. The guest can read them in a mirror.  I make t- shirts for each guest that say "Max's 6th Birthday"  on the back,  some of his friends have shirts for each year. This year I haven't decided if I will keep this on the back and write it back wards or if I will put it on the front and put the theme picture on the back.