1) While he was reading e-mails, he saw a ghost behind the computer.
2) While he was swimming in the pool, a crocodile catched him.
3) While he was walking across the street, he met a panda.
4) He was sweeping the floor, when Santa came in.
5) He was watering the flowers, when one flower changed its color.
что-то у вас про призраков много :)