Online surveys, polls, or quizzes. Consistently supporting and complementing online interactive efforts in traditional-media venues. In online media, credibility is probably more important than objectivity. We're all on information overload, so quality is more important than quantity. Also, make sure your interactive offerings make sense, are easy to use, and yield results that your audience will value.
Time is the key resource.
Interactivity always takes much longer to implement, manage, and maintain than you think it will. Also, no matter how bumpy your launch, given enough time you'll probably solve most problems. Free discussion and announcement e-mail lists. Free online survey tool. Uptilt and Bravenet offer several inexpensive interactivity tools.