Barcelona is probably a city with one of the best restaurant possibilities of Europe. When you rent a Barcelona accommodation you certainly don’t have to worry about your diner because there are plenty of good restaurants for that everywhere in the city. Going to the Ramblas is what the most tourists do but there enough other possibilities. I would recommend walking a bit further, for example to the harbor or to the areas Borne or Jaume. There the restaurants are less expensive but just as good or even better. Also during lunch the most of the restaurants are opened but for your breakfast it can be a little difficult.
In Barcelona, the people eat there breakfast kind of late, around 10/11 am. For breakfast they eat basically a sandwich, which they call a “bikini”, a croissant or a “bocadillo” (baguette). What very popular is in Spain is the “Iberico” Ham. The traditional Spanish people go to the (super) market to buy a dried pork leg. They cut of the meat themselves and eat the ham on their bread. When you visit Barcelona, you will see those pieces of meat in a lot of markets and shops. For drinks the most citizens drink a very strong small cup of coffee. For a normal coffee I would order a coffee American or simply a cappuccino.
If you rent a Barcelona apartment it would be the easiest to have breakfast included so you can be sure that it’s good. The most times when breakfast is included with a stay in Barcelona, you have a simple buffet in a restaurant in the building of your accommodation or in a restaurant which is close by. They probably serve good food like baguettes, eggs, toast, sandwiches, croissants, muesli, yoghurt, coffee, juice and things like that. Also it is the easiest way and the biggest chance you have a good start of your day.