Have students write about their own family and where their ancestors came from. They can speak to older family members and do research to learn more about the cultures of the countries where their family emigrated from and the reasons why they decided to emigrate.
Another cultural writing topic could involve comparing and contrasting their culture with another culture, either one you assign or one that they choose. A fun way to find out more about another culture and get the necessary information could be writing to pen pals in other countries. This also will get them writing more.
Kids can write about the changes that have taken place in their own culture over the years. Cultural norms are continually changing, and what was unacceptable at one point in time may be perfectly normal now. Assign kids different eras to compare with the present, or different cultural aspects to write about from the past to the present. Make the assignment more interesting by having them interview elderly individuals about how things were different when they were young.
Kids will be more motivated to write about what interests them, so offer them some choice in topics or different options in how they address the topic if all students are writing on the same topic. Making the general cultural writing topic broad and giving examples of ways to answer the topic can make this easier.