A few things I have been working on.
French Market is rich in good lunch food:
Pastoral: awesome sandwiches for $7-9 (try the Blue Pig and Fig)
Saigon Sisters: Bahn Mi for $7-8 (I like the classic)
Fumare: Montreal Smoked Hot Pastrami on rye with swiss (it's awesome)
La Cocina: Al pastor tacos for $2 each (not on menu, you have to ask)
they have several locations, I go to the one on Wells under the tracks
Flacos Tacos: Fish taco for $2.25 each (do NOT get the al pastor there)
Lunch Trucks (follow on twitter for locations)
Meatyballs Mobile (Thai'd Balls recommended with Truffle dusted chips - $10)
Gaztro wagon - been there just once...so hard to recommend anything really, but it's above average for sure.
J&J Jerk Chicken (just the Jerk chicken - dont get anything else - $8 I think) you need to find their number and call to find out where they are.