The topic that is best suited for a formal discussion is the contents of a new textbook. This is suited to the formal discussion as it relates to educational settings such as a school.
Other options such as the weekend plans of your friends, the best places to shop online for new shoes and the best cookbooks for learning to make Thai food are all informal.
My responsibility in my community was always picking up discarded trash on the side of the road. I know it's small but it really helped the people in my neighborhood. I also had to take care of the community garden. :)
