I'm sure I'll be in the majority on this, but I've often made ice cream my reward. I don't know why it seems to be the one thing I crave when I'm eating healthy, but just knowing I'll be getting it soon makes it a little more bearable.
I will say that I think the portion of the reward has to be very small. It wouldn't do any good to work hard at it for a week or two and then go to a fattening buffet to under everything.