I just remembered another one. One time, my family met up with one of my uncles and his family for a week in... Chicago, I think. A cousin who is around my age and I spent that entire week trying to convince her little sister that we were both actually aliens who had essentially infiltrated the family to learn the ways of humans so we could all infiltrate Earth and take over. My cousin and I never actually planned this, we just kept going along with each other. We never broke character and talked in great detail about our alien society and technology. We also tried to convince the younger cousin that we were going to take her back with us for experiments because she had found out too much. The only time we actually told my younger cousin that we actually were not aliens was just before we parted ways on the final day of the vacation. She was terrified of us that entire week and for quite a while after that.