To engage visitors and give them a reason to come back to the site, they must see visible evidence of activity on your site.
Create a trigger page within your site - when the page exists, the contents of it are shown in a news alert panel, when it doesn't exist the news alert panel disappears.
Post news articles, events, promotions, etc., in to a [blog]. You can usually customise the design of a blog to make it look more like a formal newsroom - in most cases just putting the organisation logo and colour scheme will do the trick.
Create a content pool (in the case of a wiki this can be either a space or a section of the heirarchy within a space) and randomly include either a whole page or an excerpt from a page within that pool.
Display a hyperlinked list of recently updated content, optionally categorised and optionally showing the author and time of update.