I'm surprised you haven't got an itinerary yet - they're worked out literally years in advance. If you know the name of the ship itself, or the line it belongs to, you should be able to find the website and check out the route; and it will also tell you what excursions are available. They're usually on the expensive side - say �50 for a half-day - but it depends entirely on how far you go, what you see etc etc, and it's very often possible to just walk off the ship and do your own sightseeing for free, or to hire a taxi/minibus on the wharf for a lot less than the ship will charge.
Look on sites like
http://www.cruise.co.uk and do a search for your ship; there are dozens of tips, reviews etc. You can do a search for individual destinations too.