Morning:
You can visit two buildings by Antoni Gaudí, Parc Güell and Sagrada Família.
You must get up early, because at 10h you must stay in Sagrada Família, an you can visit it, when you finish to visit it, you can take the bus and you can go to Parc Güell, in Parc Güell you can have a picnic.
Afternoon:
At 17h you can leave park Güell and go to Barcino, it's the Roman's city of Barcelona, it's next to Barri Gothic.
In Barcino you can see the barcelona of IVCentury. I think that is fantastic.
Afterwards you can go to the hotel to have a dinner.
Night:
At Night yo can go near you hotel, to some bar or some pub of Barcelona.