You can still book a cruise after online check in closed, on the cruise lines website or via an agent. It’s not a legal requirement to have It done and they can do everything for you at the terminal. I Booked the day before and had no problems, no check in time means turn up when you want 😉 cabins left can be the ones people have upgraded from and are then vacant. I booked 3 days before on another and got a great balcony room assigned. Not for everyone I understand, but it is possible to get a great price, cabin and cruise last minute.