All the objects must be related in a way that you have decided previously and not shared with the students. For example, you might decide that all students bringing sports equipment can come on vacation, "I'm bringing a soccer ball on vacation, what will you bring?" If a student says "I will bring a book" then unfortunately they can't come but if they say "I will bring a tennis racquet" then they can come!
Once you have told the students the object you are taking, it's time for the first student to make their guess. Begin with them standing up and allow them to sit down once they have chosen an acceptable object. It may take a while before they start catching on but if they listen hard then they'll eventually realize what kind of objects will be accepted.
Make the first round easy so they get the hang of it and then increase the difficulty, if you have a large class then you might need to let them choose 1 or 2 friends to bring on vacation (i.e. sit down) with them to make the activity move faster.
Here are some examples you might like to use:
Object has to be a type of food - Acceptable objects include bread, banana, pasta, pizza, ice cream, egg, sushi etc
Object has to use power - Acceptable objects include computer, ipod, tv, hair straightener, microwave etc
Object has to be small - Acceptable objects include key, pen, pencil, ring, pin, coin etc
Object has to have four letters - Acceptable objects include book, shoe, star, desk, seat, rice, milk etc