Walk around the city long enough and you will inevitably come across a Vietnamese sandwich shop. They offer a variety of meats served on a French baguette with cucumbers, pickled carrots, cilantro and some sort of chili mayo sauce.
If you see one, I recommend stopping and trying something on the menu. The price range is typically between $6-$10 and they will definitely fill you up.
My go to spot is Num Pang right next to Union Square – 21 E. 12th Street. I usually stop by after visiting the Green Market in Union Square on most Saturday mornings. Basically, I buy groceries for the week but then decide I should get a sandwich. I usually eat it en-route to the subway and end up with a fair share of chili mayo on my shirt.