Zona Natural serves good wholesome food in a simple and friendly atmosphere.
I ate there many times while I stayed in Rincon. For me it was the best lunch/dinner place in that neighbourhood. I loved their burger!
You can also order fresh young coconut water from them by the gallon for a very reasonable price.
They also have raw food and living food on the menu. They are located very close to the Ann Wigmore Institue (which teaches people how to grow and prepare and utilise living food), and the inspiration for this must come from there.
The staff were really friendly, and so is everybody in Puerto Rico :)
It's hard to find a good selection of healthy vegetarian food in Puerto Rico, so this place is a real life-saver.