When we arrive they will issue everyone a credit card sized key that you'll use as your room key, checking on or off board, and other places on the ship.

They have a hole so you can attach them to a lanyard, but the ship wont provide lanyards, if they did people wouldn't buy the Carnival lanyards they sell onboard. I have a bunch that I've saved from various conferences that I will bring for people to use if they want to. Some of them are plain, others have advertisements for academic products or universities. So if you have something else you might prefer that. The easiest thing is to get some shoestring tie it into a loop and loop in a keyring.
I prefer just keeping it in my pocket, but I will always have pockets, some of you or the kids may not.