Don't drink the water and, personally, I avoid fresh salads unless served at a larger, reputable restaurant, sinc ethe salad makings are all rinsed in the same water one shouldn't dirnk. Additionally, tacos and other fare sold from small carts on the street can look very tempting... but don't you dare. There are no food laws and inspections that we here in the U.S. and Europe are used to... you are on your own.
Do try, however, Mexican hot chocolate... Un taza chocolate' caliente, por favor will get you a cup (in most places) of real Mexican chocolate that's totally different from the standard, watered down versions... rich, steamy and creamy (although the cup is usually smaller that we have)...
Best of luck! Buen viaje!...