DIY - Walk down the Ramblas and people watch, buy some gifts, enjoy the human statues and then go back to the ship to eat!
This is Valencia - Go to Tourist info in Port Building - book tickets for city of Arts. Take the 23 bus from outside port to City of Arts - make sure you see a file at IMAX cinema and the dolphine show. Allow up to 6 hours to enjoy propoerly. Food is reasonable quality and not expensive.
Do the tourist bus ride (18 euros) and there is little else to do unless you like caves!