This will really depend on what your project is, and what your market might want - everyone's different!
Try to think of things that you can offer regularly, that won't cost you much (or anything!) and is not too time consuming.
Some of the most effective rewards we've seen have been;
- Bonus content
- Early access to content
- Access to you as a creator (ie. casting your editing eye over someone's writing)
- Ad-free access to your content
- On air/on stage shout outs
Be creative - even survey some of your closer fans to see what they'd want from you!