Nearer the time Cunard will advertise a full itinerary of trips - usually about six per port of call. The issue of whether you book an organised trip or arrange things yourself depends on cost, your preferences and the actual port of call i.e. are there sights in easy access of the ship. The big advantage of ships tours is the connvenience, the fact that you get a guide who should optimise the event and the fact that you don't have to worry about missing the ship. The big disadvantage is the cost which obviously depends on your budget. My advise is to surf the web and decide what attractions you would most like to see in each port of call and then work out how far these are away from the port. If they are say over ten miles away I would favour the option of an organised tour, particularly if you understandably are a little nervous at the thought of doing things yourself. On the other hand if the attractions are in walking distance of the ship or you can easily get a taxi or a sightseeing bus at the quay then think about doing things yourselves but .... always aim to get back at least an hour before the ship sails and thus give yourself a buffer as it is not at all uncommon for ships to sail without late passengers.
Their tours are well organised and informative, they usually run a shuttle bus to the centre of the town at the ports approx every 20 mins explore on your own as well its fun and easy.