For a site like Seasoned Advice, provide copies of a book on cutting edge techniques like Modernist Cuisine to some of the top users (and at the price tag of Modernist Cuisine it won't be many). You could also pick a more topic-specific book such as Under Pressure. Ideally some of the needed tools would go with the book.
How would you pick the top users? I can think of two ways to start. 1. Pick users who are active but not in the tags related to what you're giving away (so if it's Under Pressure the user can't have anything in [sous-vide]), so they will be expanding their repetoire. 2. Pick users most active in the relevant tag to hopefully expand their knowledge.
Ideally provide a part of the user profile for those users that shows them quesions and answers deemed to be related to their winning of the book.
For a site like Writers I think we most need to garner more experts on the site. A contest where you bring in an expert (defined for writers as someone employed in the industry or published by a publisher, perhaps) would be very helpful. Getting an expert to answer one of your questions or ask one of their own. We'd want a way to link people who sign up to "expert" status and then to link them to the person who referred them. There should be incentive for if those experts stay around.