Enhance the site so that visitors can directly interact with it or among each other, enabling things like user-generated content, comments, voting, chat, or forms for data collection and interaction.
The Drupal module LinearCase allows ClosedQuestions and other documents to be ordered in a "case study". The user is not allowed to navigate further than the last question he has not yet answered correctly, but is allowed to navigate backwards freely.
This is a simple little module that will get the number of Facebook 'Likes' for the nodes on your site. It then exposes that information to views so that you can, for example, sort by it.