They should be sending you a big folder full of information and other crap.
When I last took a Carnival cruise back in the late 80’s, we had at least 1 formal dinner night per 5 days. The other nights were nice pant or dress IIRC.
My recent experience was on Holland America, so I’ll share it strictly as background that may apply.
We had 2 formal nights, 4 nice nights, and 3 casual, I think. It meant quite a bit of extra packing, which kinda sucked. Especially for the men.
Your room steward is your freind and personal valet. Cleaning, laundry, special requests- give them all to him and he will make it so. Be sure and tip him well at the end of your cruise. The maid too.
There is usually a spa onboard, if you want any treatments on sea days make the appointments the second you get on the boat, otherwise you will miss out. Appts are easier to get on good port days, when most people will be off the ship.
You will be able to eat just about anything you want, at any time. Enjoy or restrain yourself, depending on your situation. Usually, the dining room selection can also be delivered for in-room dining.
Take a few port tours. They are usually led by competant guides and you will get to see a bit of the local stuff.
International cruises mean duty-free shopping. Go for it, the savings are significant. iF you are in St. Thomas, I highly recommend Irish linen, Bay Rum aftershave, perfume, etc. Booze and cigarettes are very cheap duty-free if you or someone you know indulges. Read the Customs information provided by the cruise line very carefully BEFORE you shop to avoid paying a duty on overages when you return to the US.
Most newer ships have a wide range of activities. Check out the daily schedule which should be delivered to your stateroom each night. There are also movie theaters and in-stateroom cable.