We didn't book any excursions with the company itself we did them all online before hand. I was lucky in Barcelona because a family member was working out there so they showed us what was worth seeing in the city. Barcelona, has too much in it to see in one day. My advice is, if you want to see as much as possible get yourself on a tour bus that takes you round the city. They're a good price. Also, try to leave as much valuable stuff on the ship as possible. As most people know Barcelona has quite a problem with pick pocketing, although it is improving it is still a risk. Never put your mobile phone down on a table at a cafe unless you never want to see it again. It isn't a case of being scared, just vigilant. The tube system in Barcelona is extremely good as well however it does need some planning if you're not able to speak Spanish.
The port in Ibiza is in quite a funny place. It is about a 25 minute walk away from a beach. Or your only other option is to get the bus which the cruise ship provides at a price of $10 per person into ibiza town where there is more going on.
Gibraltar was not what i expected at all. It's weird going to such a different country yet using the same currency and it has all the same shops such as marks and Spencer's and topshop! It is extremely small and you can walk from one end of the middle to the other in about 20 minutes so there's no worry about going too far away from the ship. The main attraction is 'the rock' and i suggest that if you have never done it before then by all means do it however try and book tickets online because otherwise you can find yourself queuing for a long time in the heat.