Maverick Sq. places?
Meridian Market has some of the best sandwiches in town and it is only 3 blocks from the square. They have the meridian special which is great. I would also suggest a place that is over towards Chelsea, maybe a little far from the square, called Pollo Chelean(might be something like that?) which has some of the best tasing chicken I have ever had. it is very juicy and has an incredible flavor. I will check the exact name for you on Monday....I realize it wont help for Sunday, but maybe for the future.
Taco Mex for tostones and pupusas. Of course Santarpio's for pizza (only a 5 minute or so walk). There is an intriguing bakery in the square, with lots of stuff that looked unique and tasty. Unfortunately, the staff didn't speak good enough English to explain what things were.