Invite lot of friends or the whole school room, or preschool room or whatever activity group they may belong to.  Wow, if you can keep it a secret from the child--or adult--how surprised and thrilled that person is!! Have their favorite kind of cake.  Lots of balloons, hats and party favors, but they are sooo totally overwhelmed with the surprise that you don't really need a theme as   everyone is talking and laughing about how you kept it from them and all the things you did to keep it a secret.