We used this as a creative jumpstart in a meeting this morning.
Think about what you do at work or as a hobby and try to come up with what job or activity would be the exact opposite of that.
You can just pick one aspect of your job and come up with another job that has a complete 180 degree aspect.
Example:
I work with computers in a very interactive way, the opposite of that is I would work as a Conservationist who only observes animals.
Have fun!