On almost any street in the old town you will come across little taco stands. Generally family run, and usually serving basic but delicious tacos with several toppings and usually wrapped in a fresh corn tortilla.
The food is totally local, it’s made fresh, just as you stand there, and the standards of cleanliness are high. Check it out first, before you try it — a good sign is lots of locals lining up. El Guerro, at the top of Calle Cardenas, is the best place in town for fish tacos, so good I sometimes dream about them when I am home in the cold north.
Wednesdays there is a ceviche festival down by the pier, with several vendors offering different riffs on ceviche.