Spa treatments on board are popular, reserved for 18+ guests, and will definitely fill up, so make sure to figure out the package you want, and book it ASAP — ideally before you embark on the trip.
You can still arrange one once you get on board as well, but you may have to work with them on a time you'd like. If you want to save cash but still enjoy the spa atmosphere, book access to the spa center (on the Disney Fantasy and Disney Dream, it's the Rainforest Room) and enjoy hot tubs, water therapy, and a relaxing atmosphere for the day — all for a way lower price ($16!) than the actual packages.