One possibility is to give them something for various activities on your site. A great plugin for making this reward program simple to manage is called the Cube Points plugin for WordPress.
For example, you could reward someone with 5 points every time they login to your website. You could give them 10 points for every comment they leave on your blog posts. You could even them more points for leading discussions in the forum or producing more in-depth content for the site.
What they get for the points is totally up to you. For example Cube Points makes it really easy for to you recognize the most active members on your site. You can even set titles for each level of activity. For example someone with under 100 points would show up as a “Newbie” and someone with with way more points could be seen as an “Expert.” The titles and the number of points required to earn the title are all set by you.
If you really want to get creative then enhance to reward system by offering top point earners free access to some of your premium content or maybe even give them something more tangible like your website logo shirt or some other prize. The sky is the limit.