Check out this thread as it has a lot of great ideas that you could incorporate into your surprise:
http://www.disboards.com/showthread.php?t=2342599&highlight=outline
You also might find some things in the "what do you make thread":
http://www.disboards.com/showthread.php?t=2179996
I'm not sure when you are going to surprise them, but if it isn't the day of...you could make Mel's (Bababear's) countdown chains.
A fun idea is to get a box and wrap it with a disney wrapping paper (or a solid color with a coordinating ribbon like red and yellow for the colors that Mickey wears) and put helium balloons inside of the box so that when the kids open it the balloons float out like in the commercials. Then you could attach the letter to the bottom of the string on the balloon. Just make sure they are indoors if you do this...otherwise, the balloons will float away! Then you could put a countdown chain inside of the box that you can hang up in the house to count down the remaining days.
Go to the Dollar Tree as they have tons of great Disney things like activity/coloring books, disney crayons, markers, pencils, figurines, night lights, etc. that you could add to their gifts.
I also love giving everything in a backpack so that they can use it for their trip to store things in and to keep them occupied in the car/plane.
Check out my 4shared file as I have travel games that you could make to keep them occupied. You will want to laminate them and use dry erase markers so that you can use them over and over. Here is a thread that better explains them and has lots of travel ideas:
http://www.disboards.com/showthread.php?t=2191711
Make an autograph book and an epcot passport for each child for their gift. Natalie (Donatalie) has great ones in her 4shared file. There are links to her things in the "what do you make thread".
That should give you some ideas and keep you busy. Hope your trip is magical!