Check out La Isla Bonita on Lincoln closer to Main, between 3rd and 4th. Every day for lunch except Thursdays. (And sometimes they're randomly not there too.)
Get a ceviche tostada, shrimp tostada, and three tacos. Perfection. Just watch out for the pepper on the taco plate -- that mother is H-O-T.
I'm done with La Playita. Used to be one of my favorites but they've changed over the last few years. Still need to check out La Oaxaquena.