If you want the easiest combination, add
Casa Milà first. For a denser
Passeig de Gràcia architecture stretch, continue to
Casa Batlló and
Casa Amatller; if your priority is one bigger city icon, walk on toward
Sagrada Familia. Try to pick one main interior visit instead of stacking everything, because the difference between a satisfying route and a tiring one in
Barcelona is usually one stop too many.