A Regular Quiz
Tests of knowledge tend to work well.
They're easy to create. They're fun to participate in. They can be a great source of activity.
They can also be a great source of quality information.
Even if you know the answers, put together many of the regular questions asked by members with testing questions about products/services/experiences into a regular quiz of 10 questions. Give people 24 hours to answer it. Let the winner have bragging rights.
If you like, alternate between multiple choice and more open-ended questions where you get to pick the best responses.
It sounds like a frivolous activity. Yet these regular sources of activity, traditions, and shared ideas are exactly what helps develop a community.