Well, Mexican food won't necessarily send you running for water or the bathroom. Many find that the food is simply something that you have to get used to. While you may want to stick to the nicer more reputable sit down places, you'll see plenty of gringos eagerly seeking out food stands. After all, the stands are still a zillion times better and cheaper than Taco Bell. If you opt for fresh fruits or vegetables, be sure to wash them with distilled or bottled water.
Shopping Stay away from the knock off name brand goods.
That's stuff you can get at a swapmeet for the same price. Open market style stores have great cultural souvenirs. It's also easy to find inexpensive liquor and cigarettes. As you walk the streets, vendors will call to you. Bargain with them.