Selection of bento at Soraben Kobo.- T1 2nd Floor Lobby
https://www.tokyo-airport-bldg.co.jp...ants/store/92/
Chirashi style boxes seem to be highly rated on Tabelog - but there's a wide selection if those aren't to your taste.
https://tabelog.com/en/tokyo/A1315/A131504/13123513/
Airside pickings are less attractive. A sandwich from the airside cafes etc. fills a hole but doesn't provide something "interesting" IME.