I love The Economist's weekly debate.
They select two people with different views on a topical issue and let them debate the issue throughout the week. It's well designed, structured, and moderated.
This week they're discussing whether Obama deserves to be re-elected.
Each side is offered opening remarks, they are then each allowed a few hundred words to respond to the other (evidence is good here). THroughout the week, members can contribute their own opinions and vote on whether they are for or against the issue.
For a community, the opportunities here are interesting. First, you can reward your most active members by letting them. You could also invite top bloggers/influencers/VIPs that are likely to refer others to join. Perhaps you could even let a customer debate a staff member?
Second, you can resolve real divise debates by letting members properly debate and vote on the issue. It's a shared activity all members can vote in.
Third, it positions your community on the cutting edge of the major issues. This may increase growth and the level of activity. It's also an event which members can look forward to each week, suggest debate topics, put themselves forward, and review the history of debates. You can even keep track of a member's voting record.
If you're looking for a simple idea to increase activity, a structured weekly debate might just work.