No indiscretion here 😉
I’m boarding on November 9th in Marseille on the MSC Splendida, heading to Genoa, Naples, Palermo, La Goulette (Tunis), and Barcelona.

To answer your question about the age of the ships, I’ve never sailed with Costa, so I wouldn’t be able to give good advice.
As for MSC, the fleet is quite varied—from the oldest ships, which are also the smallest, to the newest ones, which are big vessels. It’s certain that the latter will offer more options for entertainment, bars, and fun activities on board. But the older ships are far from outdated or lacking in activities—quite the opposite, in fact.
Really, everyone can find what they’re looking for based on their expectations on most ships.
For me, when I choose a cruise, it’s a mix of the stops and the ship that determines my choice. If a route has more sea days, I’d lean toward a slightly bigger ship.
On the administrative side, you summed it up well: valid ID and passport.
Personally, depending on the storage space, I always leave either my ID or passport in the cabin safe. That way, I’m sure I’ll always have ID available if something happens during a stop.
As for tips... it’s hard because there are so many topics—you’ll have to guide me on what questions you have.
A first one, though, about luggage. I always pack a mix of outfits to be comfortable during stops and slightly dressier clothes for on board. I also always bring a backpack. It’s handy during stops and on embarkation day.
When you board, you drop off your luggage at check-in, and it’s delivered to your cabin later in the day... with no exact time given. The backpack lets me keep a few essentials with me, including a swimsuit in case I feel like taking a dip as soon as I board 😎
As for excursions, I rarely book the ones offered by the cruise line. Since many ports are in the city, I usually just head out on my own and wander around a bit randomly. That way, I get a taste of a destination, and if I like it, I’ll come back later on my own to spend a few days and explore deeper.
But on that note, everyone has their own way of traveling.
In the end, whether it’s excursions or onboard activities, everyone finds what works for them based on their own expectations.