Bar Milano, right at the Marcel, serves breakfast.
Another option is Penelope, on the corner of Lex & 30th St.
Though not breakfast, I have eaten at both places, and the meals have been very good.
Edited to add: The atmospherics are very different. Bar Milano has separate bar and dining rooms with sophisticated modern decor. Penelope is a small, sweet place with kitchy decor.
Not sure when the OP is going, but if you are going on a weekday, Bar Milano only serves breakfast on the weekends now.
I also agree with both Penelope and L'Express, esp. if its a weekday so you can avoid the crowds. The hotel is 24th and 3rd and both places are within a 5 minute walk from the hotel. If you want a great bagel, Ess-a-bagel is on 21st and 1st.