You could do a grand welcome for your hotel/resort room and include a fun "package" surprise for each of the kids.
Here is a thread with some information:
http://www.disboards.com/showthread.php?t=2342599&highlight=footprint
http://www.disboards.com/showthread.php?t=2623412
Is the teen a boy or a girl?
For a girl, you could make certificates for fun things like the "pick a pearl", maybe a spa experience, or some fun shopping certificates for downtown disney. If she is crafty, you could put together a scapbook kit with lots of funpapers, stickers, etc. and give her a camera (even a disposable camera would be fun).
For a girl/boy you could plan a fun day at Epcot and do an "Amazing Race" for them. This is a lot of work, but it could be something special for them. Here is a thread with some information:
http://www.disboards.com/showthread.php?t=2063192
You could also buy the "hidden mickey" book and give them a goal of "X" number of hidden mickey's they have to find throughout the trip to win a prize. Make a sheet where they can track what hidden mickey's they found and where they were.
You could make autograph books, or an epcot passport (Natalie (Donatalie) has a GREAT one!) Teenage boys will probably not care about either of these, but depending on the teenage girl she may love or not care for it. You know your teen well enough to know if he/she would like it.
For a boy, you could give him a gift certificate for a mouseracer (I think that is what it is called)...it is a miniature boat that they can drive around a lake at one/two of the resorts. I have a coupon for this and several other things in my 4shared file in the link in my signature.
Check out this thread as it may give you some other ideas...it is a "what do you make" thread:
http://www.disboards.com/showthread.php?t=2179996
Also, you can give the teen a trip planner and have them help you plan the trip...especially things that they want to do.
One final thing...you can put together a cool backpack with travel things for the trip and/or things they can use while at the parks. I.e. snacks, crosswords for during the travel, music to listen to, etc.