Three full days is right for Barcelona; four with Montserrat

Day 1 is Gaudí: Sagrada Família, then Casa Batlló or La Pedrera

Day 2: the Gothic Quarter and El Born on foot, then Barceloneta

Day 3 is Montjuïc: Miró Foundation, the castle, the best free view

Two days: drop Montjuïc and pick Park Güell or El Born

Montserrat is about an hour by train and needs a full day

Five days or more makes Barcelona a base; the full split in the guide