My wife and I have cruised as a couple on Royal Caribbean to Alaska, and on Carnival to Cozumel/Yucatan. We also took one cruise with a grown daughter to the Bahamas. Excursions were the fun part.
To me, the cruise ship was mostly a floating hotel. The ports of call were the real vacation. We did a lot of excursions on our Alaska cruise, and stayed extra time at a B&B in Alaska. For our Cozumel cruise we did one excursion (dolphin swim) and then went out on our own on the mainland. We spent an wonderful day snorkeling and enjoying the food. There are extra perks on the ship - shows, good food, etc. But, that is not why we cruise.
Our next cruise is a change in our approach. We are taking the grandkids on a Disney cruise. We are not getting off the ship in Nassau, except maybe to get some souvenirs. We definitely will get off at Castaway Cay and enjoy a beach day. For this cruise we want to experience Disney and all of what Disney offers on board the ship. Lots of memories to make with the grandkids.
But, after this, we are backing to cruising as a couple, exploring ports of call. We are hoping to use the NCL Pride of America cruise to visit several of the Hawaiian Islands. The ship is definitely just a floating hotel, and overnight transport between the islands.
So, if you like to visit different locations and not worry about transportation or hotels, cruising can be a fun way to have that experience.
OTOH, we have good friends who love to cruise a lot and almost never get off the ship. They see it as an inexpensive way to relax and get away.
So, it all depends on how you want to define fun.