This city is absolutely amazing, stunning, beautiful, bustling, vibrant, fascinating - I could go on.
We decided to DIY it. We had the whole day here and boy oh boy did we cram it

The shuttle bus dropped us off at the World Trade Centre at the bottom of Las Ramblas near the Christopher Columbus monument. Once at the monument stand with your back to the sea and Las Ramblas is across the square in front of you. A very short walk up Las Ramblas you will see on the left the entrance to the Metro (a red M on a white background) this is what we used to move around Barcelona and it is so easy. Firstly you will need cash to get your tickets, there are no staff kiosks it is all automated but don't worry there is an English option. We purchased the all day travel ticket which was 7.9e each same price for adults and children. When you first go through the barriers the ticket does take a while to register so don't worry if you don't get it back immediately after the first use though it is very quick. We made our way to Park Guell first which was amazing, it does get very busy with tourist buses and school trips but still an amazing place. We spent ages mooching around the Park and sat and listened to the amazing harpists and violinists around it. From here we walked back to the Metro stopping at a super food market where some local chocolate and walnut flans were order of the day. We used the metro to travel to the Sagreda it was very busy with long queues waiting to go inside so just walked around the outside - and it is beautiful. Fascinating contrast between the original Gaudi part at the front and the more modern architecture at the back. From here we decided to jump back on the Metro and went to explore the Gothic Quarter - just fascinating exploring all the narrow streets and a compulsory visit to a small bakery was made to purchase some churros mmmmmmmm yummy. Quite by chance whilst exploring the narrow streets we found the amazing Gothic Cathedral, and what a beautiful find. There is a charge of 3e each to enter but what meets your eyes is breathtaking, it was a beautiful sunny day which made the colours lighting up the floor from the stunning stained glass windows even more amazing. At one end there is a huge 10m wide Rose Window and then down both sides of the church there are huge arch stained glass windows. We were allowed to explore the crypt and there was also a fascinating museum and walled garden too. From here we walked back downLas Ramblas to the Christopher Columbus monument and then crossed over the road and turned to the left where there a a lovely marina which we walked around before jumping back on the bus and returning to the ship. An amazing city - cant wait to return some day.
If you feel uneasy about DIYing the day before we were due to arrive at Barcelona a bus trip was advertised in the horizon, it was priced at £19 per adult and I think £12 per child. There was no tour guide but the bus dropped off at Park Guell and then at the Sagreda, (entrance fees not included) but you are only given a fairly short time to explore - about 40mins according to a lady we were chatting to. Park Guell is free to enter and at Sagreda there are different admission fees depending on what you want to do.