I would additionally highly recommend you to close your pockets, hide all cameras and worthy stuff while waiting at the almost large entrance crowd at the sagrada basica caused by pocket thieves.
To be honest, the entrance fee is ridiculous and I refused to pay. Las Ramblas are ok just to see, the prices there are much too high, tourist terrain
Gaudi museum is very cool, be sure to arrive there before 9AM, later on lots of busses there are spitting out countless hords of tourists and its getting there quite crowded. Use the underground, simple and cheap.
Dont miss out the marina, grocery hall and local food.
We had our stay very outside of Barcelona in a location with name Castelldefels, hotel was one of the Best Western group and it was just a few minutes from the train station. Not very far from the beachline, nice small taverns into it and travel time to central station round about 30 minutes.