hi bigfran
the hop on bus is probably the easiest and cheapest way of seeing the city sights, 18 euros for a two day pass which is about the time you need to see MOST things. You can get on and off at certain places and change lines in differant directions of the city, very good value for money as walking is probably out of the question to see everything,its also good if you need to go to your hotel,harbour,top of Las Ramblas and your weary legs will not take you,just jump on the very frequent service and enjoy the ride resting those aching limbs.
enjoy a fantastic city