The Frank Stella's are the best. They are all over the city and they always look amazing. Two of my favorites are the Saatchi and Saatchi lobby (pictured) and the Citi Corp building in Midtown.
Part of the Marlow restaurant group, Diner is one of the more casual / rustic options. The menu changes daily and is written out (most likely upside down by the wait staff) on your table or the back of a receipt. The only constant item is the burger, which to be honest, is one of the best in the neighborhood. If they have a breakfast sandwich for brunch, get that. The fried chicken sandwich (with dark meat) is also a favorite. They make scrambled egg dishes better than most. Dinner is solid all around (they crush a steak for two). Homemade ketchup and dijon mustard are on every table, so regardless of what you get, these two condiments are worth the visit.