Norwegian Cruise

How active are you? Are you comfortable walking short and longer distances or like to have a taxi or bus? What are you specifically looking to see and do at each location besides the one beach day?
From an excursion point of view, the three points I'd make when weighing the pros/cons of getting them via the cruise line are: 1) more expensive; 2) "guaranteed" and prioritized by the cruise line for no "miss the ship" worries, and 3) large groups. Exploring by yourselves is a bit more work, but totally "your time" and pace. If you have a ship credit, it makes sense to use it, but to use it wisely on the one or two excursions that would be most challenging to book or do on your own.
Santorini is the whole island, and the ships generally park offshore and tender in to the docking area. When we went - several years ago - the tenders dropped us at the "Old Port" below Fira, and our options were to take the cable car (6euro each way) or walk the old donkey path (free). We chose to walk up and then cable car down. Probably easier to cable car up, and walk down :) I think all cruise ships still use the Old Port and the tendering.
But, sort of knowing more of your hopes for the islands can help give you more specific ideas.

I see that Paros is becoming a cruise destination, and now with 2800 passengers ships.

It had become more and more overrun with tourists in recent years, in the next few years it will become really unbearable for Paros lovers.

Naxos, next on the list?

Have you read Rick’s Med cruises book? If not, I highly recommend it.

My reply is based on not taking ship tours.
The ship likely will be docked in Rhodes very close to Rhodes town. As I recall we could see over the city walls from our balcony.
Just explore Rhodes town yourselves. Its a great experience. No need for a guide.
Here are some images of Rhodes. Just hit the arrow on the right side of the photo and the next photo will come up.https://www.flickr.com/photos/stanbr54/22975650735/in/album-72157634754696058/

Mykonos. I think that might be a tender. You have a couple of options. Take a ferry from Mykonos to Delos. Its a great ancient site. Its a short ferry trip and not expensive. You will have to do some research on DIY. As I recall you have to take a water taxi from the dock into Mykonos town where the excursion ferry takes you to Delos.
If you want to do something else just wander around Mykonos town. Its fairly small and has lots of charm. It is also very expensive, not the place to buy a souvenir.
Lastly take a taxi or local bus out to the beaches. The bus service is good and regular.
Day trip to Delos and Mykonos https://www.flickr.com/photos/stanbr54/albums/72157637922138466

Like Jo Loui I am saddened that Paros is on this itinerary. Its small and a wave of cruise ship tour people probably will overrun it. Just wander around the town. Perhaps take a taxi or water taxi to Antiparos or the great little port at Naousa.

Crete is large. You will likely be docking at Heraklion or at Chania. If its Chania then just take the KTEL bus (or a taxi)from the port into town. It has a Venetian era old town with city walls and a lovely harbour. You can spend the whole day just wandering in the maze of back alleys and along the harbour.
Chania 2022. https://www.flickr.com/photos/stanbr54/albums/72177720299646015

If its Heraklion you will be docked close to the Venetian harbour where you can visit the fortress and Venetian buildings in the area. Later you can walk up into central Heraklion which has lots of interesting buildings and shops. The central are is nice to wander around. You will have to navigate the heavy car traffic to get to the much quieter central area.
Castello Heraklion https://www.flickr.com/photos/stanbr54/albums/72157686964935102
Heraklion https://www.flickr.com/photos/stanbr54/albums/72157655935354556

Santorini as noted is the name of the island. Fira is the main town. Its a tender port so you as already mentioned you have to either walk up 600 steps to Fira or take the cable car. It would be a good idea to check to see how many cruise ships will be in port that day. You might have 10 to 20 thousand people competing for seats on the cable car.
An option would be to find a boat at the harbour that takes you to Oia. There may be more than one doing that trip. You get a sail around the caldera and land at the lovely village of Oia. After visiting Oia see if there is another water taxi going back to Fira. If not take the local KTEL bus to Fira, spend a few hours walking around and then get into the cable car line up. This is not a port where you can plan to get back to the ship quickly.
An option would be to take a ships tour. They tender you to the ferry port where buses meet the tours. I was under the impression that the tour buses drop you off back at the ferry port but recently I read that the tour ended in Fira to take the cable car back down.