I am using this activity to emphasize to my first grade students that the main idea is more than just one word! I picked out 2 books for each partnership that were on the same topic. Some of the books had the same or similar main ideas, whereas some had different main ideas. (i.e. 2 books about rain but one's main idea is "things you see when it rains" and the other's is "clothes you wear when it rains".) Students read the books together and used picture clues to help determine if the main ideas were the same or different.
I hope it is helpful to you!