Both Emporium and Emquartier on the other side of the BTS have food courts. Emporiums is on the 4th floor, Emquartiers in the basement. But the star of Emquartier is the Helix Tower restaurants from the 6th to 9th floors, above the excellent roof garden, where there are, I believe, about 60+ restaurants, all based around a ramp you walk gradually around taking you up the various layers. Escalators and lifts available if needed. Everything is available there, although a sad loss is the fish and chip shop, which I thought excellent. Went there a couple of weeks back and it's gone.
There's also a great steak restaurant on the corner of soi29 with Sukhumvit, which is halfway between T21 and Emporium / Emquartier.