Good port to visit. Don't be afraid to use Metro, gets you around to main tourist areas away from traffic jams. Right next to Sagrada Familia, one near to Magic Fountains (Placa Espanya)which you should see as it starts to go dark for best effect. Worth a trip to castle at Montjuic via Funicular - get to this via Parall-el Metro station - very simple when you get off metro train. Nou Camp worth seeing whether footie fan or not.
Easy to get to Nice on port side of railway or Monaco across the opposite side of the station. Big problem if many people trying to buy tickets at machine, which only took credit cards. Impatient people caused machine to malfunction so train arrived and many ticketless people! Suggest buy tickets on far side of station before climbing all the steps. Trains very good and on time - many double decker.
Train from Civitavecchia good when on time! Read port reviews by Tom Ogg on Google for how to find station - easy walk along port road 10 mins. Beware - check and double check that you get early enough train back from Rome as trains are often cancelled without warning. Also get correct ticket for train type ie:Local or Inter City. Lots to do and see - research well and realise you can't do everything in one visit due to long queues at most popular venues.