Here is a great way to increase activity in a community; tell your members what they are doing.
People want to do what they see others doing. If you tell them what members (like them) are doing, they're more likely to be involved.
If members are discussing something on a forum, it is good practice to write a post telling the community that this topic is being heavily discussed on the forum. It accentuates the level of activity.
If several members have signed up for an upcoming event (through your community or elsewhere), tell your community about it. They might arrange a meet-up.
Frequently tell the community what they are doing.