Most ports will have many ships visiting at the same time. Each ship has 2-3000 people on them. This means the port itself (often leading straight into town) and the quay will be VERY crowded. The shore excursions therefore can be less of a problem. Do some research. You will also find that there will always be an excursion with no or minimal walking which just gives an overview of the island. If you want to avoid crowds then avoid excursions which always leave at between 7.30 and 8.45 am. Go off the ship at about 10 am and get a taxi - it WILL be cheaper but agree a price before you go - and go wherre you want. By this time the excursions will have left and you will have the places to yourself and other like minded people. Dont be scared as most taxis prices will be displayed on boards at the end of the dock by the tourist board. Going with another couple splits the price in half again! Either way you will save around half of the cost of a ship excursion.
I think you both might have a better experience if you use a Taxi service to go to the places you like in the brochure, there are always loads awaiting business at the docks. It could even be cheaper and they also have things called Taxi-vans or People-carriers which you could use if you find friends onboard who might like to accompany you. It is much better, though, if you do a bit of homework for each port before you go. Regards, Ida.