Haven't been on a Carnival ship in 3 years so my advice might be a bit dated
-If it isn't your first time, skip the life jacket intro
-You have to pay for wifi
-Expect a ton of midwestern cacs and

couples who've never been on a cruise before
-The Gyms on their ships are usually pretty alright
-There's photographers all over the ship, especially before and after dinner
-Food lines are long as fukk early in the morning and around the afternoon
-You can't buy stuff on board with cash
Edit: and yeah, do a 3 day cruise, the longer you are on a boat the less fun it is
Edit 2: Heard Carnival takes your bottles away and tries to sell you their own water and soda before you board