The cruise lines with excursions included are considerably more expensive. Some just include basic 2 hour trips.
Mist of the ports you have mentioned might not be ideal for doing long trips on your own, and it is so much more comfortable to be with a few other people. On the “Roll Calls” there are often groups putting a trip together in a minivan etc. One usually pays per couple direct to the company.I have joined these in the past, and all have been great fun and value! To name a few, Columbia, Namibia & Guatemala. European ports are much easier.
Cunard trips ashore run from upwards of $50 depending where and how long. You can see what is available once booked (and have found the same with other lines).
A lot to think about, but it sounds a great voyage.