We did the same thing for our girls for the first trip in 2005. We only did it for 21 days (the girls were 3 and 1). This time, I did a little calendar w/tear off pages and pictures of the characters for each day. As they the 3DD are a bit older, they understand the count down a bit more!
We will be there at the same time Only 69 days to go
I also made a chain when I got to 150 days... it wrapped around my bookcase, over the tv and then came back around. Now it barely goes over the bookcase