Traveled to Monserrat for the morning, great trip. Having been to Barcelona a couple of times, I would use the very good metro system to visit the sights. Cheap and clean with a station close to the drop off point from the shuttle buses.
From where we were docked it was a two minute walk to the metro, we purchased an all zones one day ticket and popped up at various locations to have a look. It was a very hot day so we drifted back early however we were aiming to grab the 28 tram which goes past the main sights, next time perhaps!
We went for a walk into the town, climbed the hill to the ruined fort for some great views and strolled back to the ship before it got too hot. Nice shopping area in town and a small shopping centre right beside the dock.
We stayed in Cadiz, walked to the beach for most of the day. Strolled back, stopping off in a couple of bars for drinks on route. This is a compact port of call, some nice gardens and I know a lot of people made use of the tourist buses located very close to the dock.
Ship was docked a fair way from the town centre, the shuttle looked expensive for the three of us and we caught a cab for half the price. Dropped off by the cathedral and most of the sights/shops were nearby. Plenty of cabs for the return journey.