The small difference
On the board, draw a seating plan of the room and get the class to copy it. Each student round the room then says their name and everyone else writes it down in the correct place on their plan. Ask the class to study the names for two minutes, then put their plans away. Ask a volunteer to leave the room; while he/she is out, two other students change places. When the volunteer comes back, he/she must notice and name both students that have moved. Repeat the game a few times with different volunteers. After a few turns, make the game more difficult by changing two pairs at a time.