Divide the class into two groups. While one group waits outside, the other group looks around the room, trying to remember as many details as possible. After a minute or two, the groups switch places. The students who had been waiting outside enter the classroom and proceed to change 10 things. (To ensure that only 10 things are changed and to help keep track of what was changed, assign smaller groups of students to each change a specific number of things about the room.)
After the changes of been made, the waiting group comes back in and tries to figure out what has been done to the room, e.g. the board has been cleaned; a chair has been moved; a window has been opened/closed, etc.
I’ve had students write their guesses on the board before discussing each one.