Just like people without Down Syndrome, people with Down Syndrome have different likes and dislikes, so I would start fist with finding out what sorts of things she likes.
Some people like shows, but others prefer wild rides. If you are familiar with WDW, once you figure out what kinds of things she likes, you can use your knowledge of the parks to pick out those types of things.
Are you going for the day, or staying a while?
Are you going to one park or more than one?
Which park (or parks) are you going to?
Does she have any physical or medical conditions that you need to take into account?
If she has any heart or muscle difficulties, or problems with walking, you might want to get a wheelchair for her. That might also be a good idea if she is just not very active. One day of walking at WDW can be 3 miles to as much as 12, with 5 or 6 miles being pretty common.
If you do decide to rent a wheelchair, post 2 of the disABILITIES FAQs thread will be helpful with rental information. You can find that thread near the top of this board or follow the link in my signature to get there.
Post 1 of the disABILITIES FAQs thread is an index that tells what information you will find in each post. Since you don't have much time to get information, that might be the bet place to look.