This is an activity I have used a lot as an Activity Director.
We decide on a subject. Sometimes I give them a theme and at other times I ask the group/individual to think of the subject.
I use a whiteboard (pen and paper would work great for an individual), and then we try to think of as many items as possible belonging to the chosen category.
For example: Name as many as you can of …
States in America
Countries in Europe, Asia, Africa, South America etc.
Types of ball games
Things you can find on a farm
Items you may find in a bathroom
If you are working with people that need encouragement, count up the amount of entries you thought of, and praise the effort. “Wow, we thought of 19 things you find on a farm, that’s great! The activity should be fun.