how long are you staying in manhattan? i work at 52nd and 6th ave.
there's a heartland brewery at 51st and just west of 6th ave. they have decent beer and kind of cheap food (for manhattan). there's also outdoor seating which will be nice tomorrow (sunday).
for other places you can walk along 54th or 55th between 5th and 6th for a bunch of not expensive restaurants.
for really cheap but pretty good but no seats, there's a hallo berlin wurst stand at 54th and 5th which is supposedly worth the constantly long lines. they have really good bratwurst and all kinds of wurst. there are no seats but you can walk up 5th to the park or sit around rockefeller center or maybe on the steps of st. patrick's.
for all you can eat sushi, there's a place called funayama on 10th and greenwich in the west village. it's $25 but it's a great selection. it's only available on mondays and thursdays though.
minado also has all you can eat but it's somewhere on the west side i think but it's in midtown iirc so it should be closer to your hotel and is always all you can eat, not just specific days.
if you want the mack daddy of all you can eat, head to churrascaria platafarma at around 48th and 9th ave. it's a rodizio (brazilian bbq) which means it's a sh!tload of meat. it's pricey though, so i would only save this for a special occasion. there are probably other cheaper rodizio places nearby.
if you're adventurous enough to head around the central park area there are a number of places to eat in the 60's and 70's on the west side around amsterdam ave (or whatever the ave is that is one block west of central park west)