A new place in MetroTech Two, called La Defense, is pretty good. Also, I think DeKalb Market will re-open soon for weekday lunches -- some interesting choices, if overpriced. If you have time to walk a few more minutes, you could walk down Hoyt Street to Fast & Fresh (Mexican joint with tacos, etc.) or Mile End (Montreal-style Jewish place w/smoked meat, etc.). Also, Nicky's Vietnamese sandwich shop on Atlantic Ave. If you search chowhound for posts on nearby areas -- Smith Street, Atlantic Avenue, Montague Street, etc. -- you will get lots of ideas!
Nicky's Vietnamese Sandwiches
311 Atlantic Ave, Brooklyn, NY 11201
Fast and Fresh Deli
84 Hoyt St, Brooklyn, NY 11201
97 Hoyt St, Brooklyn, NY 11217
332 Flatbush Avenue Ext, Brooklyn, NY 11201
2 MetroTech Center, Brooklyn, NY 11201