Gibraltar is a very busy port. It is supposed to be Duty Free, but I found it quite expensive. We did not use the ship excursions, but just got a taxi from the town centre up the rock which was very enjoyable. Most taxi's charge you 25 euros each for the 2 hours trip(that's with 8 in the cab). The cable car was very busy and we couldn't be bothered waiting in the queue, it does not run if its windy.
Fantastic place, but you cant do it all in the day. be realistic as what you can do. Don't rely on the trains, they sometimes just don't run. We made it back with 5 mins to spare because we say at Rome train station for over an hour.