@kambalange, this forum post should help: When is definite article needed to contract with de
Basically, it’s about how generic or specific the phrase is intended to be. For example, “a zona dos restaurantes” is more specific/concrete, like making a direct reference to the restaurants that are specific to this town. But you can also say “Zona de restaurantes” or “Zona de restauração” and instantly sound more abstract (the area where restaurants are, whatever restaurants they might be). “Caixa de aspirinas” is generic, “caixa das aspirinas” is somehow specific to those tablets at hand.