Is the trip keyed to a specific event - wedding anniversary/milestone birthday/other? If it were, you could build something from that event.
Say, if it was a wedding anniversary, get their wedding photo and photoshop Mickey ears onto the bride and groom and put it in a Disney frame...
I personally like the idea of a scavenger hunt that someone mentioned above. If you wanted to do it this Easter, how about this idea...get some of the plastic eggs and have your brother and you write down special memories that you remember about your parents (things they taught you/things you did together/what family means to you) and have your parents look for them and then read them all together as a family.
After all of the eggs have been open and read, have three more (larger) eggs ready.
In the first one - have a blank piece of paper. (Symbolic of the memories this trip will bring)
In the second one - have a calendar marked with the days the trip will cover.
In the last one - have all of the details - the flight (if there is a flight), the hotel, and maybe a reservation or two.
I don't know if they are still available, but for Christmas a few years back, the Disney store sold a recordable gift card. You could buy one of those (assuming you are near a Disney store!) and have the siblings say somethign to the effect of "Mom and Dad, you have given us so much throughout our lives. You taught us lessons that don't come from books - you taught us about love, responsibility, community, faith and most of all, family. For all you have taught us, and done for us, we want to give something back to you, and we want to take you to Walt Disney World."
No matter what you decide, please share it with us!!
