Greens is good. I had a bun that would only eat small amounts of greens every half hour or so, for almost 2 days following his neuter, but I felt it was a sufficient amount for him to be ok for those 2 days. So if you can encourage the greens every half hour and she will eat at least a small handful each time(for a small to med. size bun), then she should be fine until her appetite returns. You should see a gradual increase in appetite and that she will gradually start eating her hay and pellets as well over the next day or two. This shouldn't go on for more than 2-3 days from the day she was spayed, or you should be contacting your vet.
I would hold off on the watermelon or any other sugary starchy treats. A small sugary treat is ok at first to try and stimulate appetite, but continuing to give sugary foods could lead to a bacterial imbalance and further digestive upset and problems. Since she is eating leafy greens, I would stick with those as they will be much better for her and less likely to cause issues. if she doesn't seem to be drinking, she will be getting some fluids from the greens, but if it isn't enough you may need to syringe her some water several times throughout the day.
If you have concerns or something doesn't seem right with her, you should contact your vet.