I've been thinking about this topic as well while consider whether to start Patreon or not. I haven't done it yet because I haven't been able to come up with many rewards ^^'
The only things I was able to think of were early access to the chapters and perhaps a sneak peek to the next chapter, like two or three paragraphs (this would be a lowest tier reward).
I saw one author write and upload their next novel completely Patreon exclusive at first and only when it's completed they will put it up on Tapas which I think is also a very good option but probably works the best when you have a bigger audience and large amount of Patreons.
Prompts could be nice like Elixiink mentioned. There could be a poll every month and you'd write one or few texts based on the prompts.
And of course there is always the option to offer NSFW content if you are comfortable with it yourself. Or perhaps AUs as a softer version of that. These could be included in the polls as well.
