Weather defines a city
For example, Fairbanks, Alaska, is colder most of the year than Sydney, Australia. Pair your students, and have them choose two cities far from each other.
Collect the data, and create a diagram
Before a Venn diagram can be created, students first have to collect data. Take a topic with diversity, such as the different eye color of students in your class. Have a student survey their classmates and record the data.