Create a feature that summarizes forum activity on a regular basis, such as daily, weekly, or monthly. You can include activity such as new posts, responses to posts, or the creation of new categories. The digest could be presented as a page on the forum that people can look over. Also provide the ability for users to subscribe to the digest and receive an email with the information based on the interval they select: daily, weekly, or monthly.
Design the digest as you would like it to be. Also, you can develop the digest portion of this feature first and follow on with how to subscribe and manage it later.