Located 2 blocks off of Playa Norte, in front of the mercado you will find a plaza of lunch shops containing 4 cafes that are all excellent.  These family run restaurants serve fresh, authentic dishes at really affordable prices.  Lots of locals and tourists alike enjoy the stands food from 10 until about 2:30 to 3 depending.  We tried #1 and #4 and both were amazing and averaged about $3 a plate.  Tacos, tortas, chilaquiles, enchiladas, shrimp soup, and moles are all above average here and I would argue better than any other place on Isla Mujeres.  Let me know what you think when you go.  What is your favorite stand? Pro Tip, after you eat go and check out the small mercado and see what all is on offer with the local stands.  There are butchers, produce stands, and even a tortilla factory.  They also have more informal taco stands in the back that even cheaper and more casual than the outdoor cafes.  Buy some fresh orange juice at the fruit stand!  Just buy it and thank me later!