Solo dining in Alphabet City
Depends on your price point.
Casa Adela is always solid for cheap Latin eats - they make a decent cubano, their octopus salad is really good, roast chicken excellent for its style.
Korzo Haus (at the SE corner of Tompkins Sq Park) makes great burgers woth some interesting twists, but forewarned they're a gutbomb - delicious but crazy filling.
Bunch of places on Clinton St - Clinton St Baking Co, Fatta Cuckoo, Tapeo 29, or get a couple a la carte small plates at the bar of WD-50 if you're feeling extra-fancy.
Slightly further (maybe a five minute walk, if that - I live in that neighborhood and go to The Stone fairly reg) is Pok Pok Phat Thai, great place to grab something fairly quick and delish.
Mercadito is on 11th and Ave B, their tacos and guac are really good
Back Forty is on 12th and Ave B and serves farm-to-table American food
Casimir on 7th and Ave B serves French food in a casual setting
Zum Schneider on Ave C and 7th is a fun, rowdy beer hall with hearty pub fare