If you visit the P&O website and search for your cruise as if you were looking to book one then enter the information section there is a detailed description of each port and an option to see the excurisons p&o offer, I find this very useful as booking excursions early is important as popular ones sell out.
I have just come back from a cruise on the Oceana which stopped at Vigo and Lisbon and certainly in both these places P&O layed on courtesy coaches to take you to the centre of town and back at no charge. There are also tours you can book if you wish to go further a field. I have to say though go up to the old castle in Vigo, its a long walk, or take a taxi, its so peaceful. And Lisbon is just gorgeous, reminded me of Barcelona.