It seems like the most common donation reward is the custom Donator tag. I've never used SMF forums before, but when I ran a forums with vBulletin I could allow users to write in their own custom title, so like you were saying, if they donate they could have the ability to change their title to whatever they want. (With obvious restrictions.) Also, changing the format of their title, so for example instead of "Their custom title here" they could customize it to, "Their custom title here". Simple things like that. I'll keep thinking about other incentives we could maybe have.
Also, thank you for the promotion. ;D
Edit: We could add in a custom forum section specifically for donators where their posts are "escalated" to a higher priority and answered by staff more promptly (When the site becomes so popular we can't keep up with every post.) Again, I'm not big on special treatment, but if they're helping keep the community alive, I think they deserve some form of "special treatment."