You can book them on the Disney website. They are added to your stateroom charges and pay at the end of your cruise. Sometimes they actually have better/more options to book directly rather than through Disney. example: in Cozumel they offer a swim w/ dolphins. actually you aren't swimming w/them you are just enteracting with them, touch, kiss, feed them... where if you book directly through the aquarium that offers this you can book the real swimming w/the dolphins. Sometimes its cheaper this way too... but it is easy to just book through disney and let them be responsible for taking you and picking you up... pros/and cons to everything

hope this helps some. there are some fantastic excursions for sure!!!