My family always goes on Disney cruises (I've been on seven) and they've come up with some really fun ways to tell us.
1st cruise: I think they just told us about that one
2nd cruise: At our birthday party (me, my sisters & my brother just had one big family party since our birthdays are 11 days apart) they gave us this group gift. It was a box with puzzle pieces in it and when you put it together it was a picture of the ship with a date.
3rd cruise: This might have happened for the 4th one not 3rd but I can't remember. At my aunt & uncles we would always play this game where everyone sits in a circle and gets a wrapped present and then someone reads a story that they made up or whatever and whenever they say left or right in the story you have to pass your present that direction. In the end the presents were all things from past cruises and we had to figure out how it related & what it ment.
5th cruise: It was the "Mexican Rivera" cruise so my parents took us out to this authentic Mexican restaurant and told us there.
6th cruise: me and my siblings "accidently" saw my parents at the booking desk while on the ship...whoops!
7th cruise: we all knew about that one, last family vacation before big bro went to college.
I loved how my parents tried to make it special every time they told us(even thought I can't remember some of them right now)...except I'm a snoop and almost always knew about it before time, hehe.
Oh, and if you ever get sick of Disney World, go on a Disney Cruise! You'll have a blast, I swear!