Passeig de Gràcia is a very important street in Barcelona because of it’s modernist buildings.
Gaudí, Domènech i Muntaner, Puig i Cadafalch are 3 modernists arquitechts from de 19th Century who build a lot of beautiful buildings. 4 of them are in Passeig de Gràcia.
If you want to know a little bit more about them, just visit Wikipedia:
You should wear comforable shoes and then just… start walking