What are the actual dates you are going- and when are you wanting to tell them? When we surprise our kids I let Daddy take them to eat and a movie the night before.We usually fly out on a Sunday, so Saturday is great for this. I spend all week making sure the laundry is caught up and the house is clean.
Then after they go to bed, I pack all the suitcases and hide them in the back of the suburban. I get them up early the next morning(way earlier than for church) and ask them if they would like to go somewhere different! It's hilarious watching the little wheels turning!

My middle son figured it out 1st last time. But his older brother said he thought Disney world, but was afraid he was going to be disappointed!!! Then they can pack a back pack with books and stuff I have already bought and hid and their game boys or whatever for the plane trip.
They really like to be surprised better than knowing-the suspense just kills them. In May, my dh wanted to tell them 2 weeks early. I warned him not to, but he thinks knowing is half the fun

Silly man- Mommy knows best!!
They were all excited for about 10-15 minutes, then the oldest busted out crying cause he had to wait so long.
dh was shocked to say the least. so guess what? The next trip I am planning is also a SURPRISE!!!