I like the ideas of clothing, music, and art supplies. Candy has been suggested but I disagree. Many people with Down’s Syndrome are overweight or have heart problems.
I once worked with a woman with Down’s Syndrome who loved all things related to John Lennon. Setting her down with a picture book about her favorite musician was a real treat.
The big thing is to make certain it is age appropriate! She is an adult!
Giving her a babyish gift might cause her to become upset. Also, if gifts are being handed out to the other people, she will feel left out is she doesn't receive one.
People with Down's Syndrome can be very sweet, or they can be totally mulish.
If your guest is not on her best behavior allow those who know her better to deal with it. An example is a tantrum after being told she can't have anymore cookies.
Don't rush in and offer her more cookies! This will only teach her that having tantrums will get her what she wants. Those who care for her on a daily basis will not be happy with you. A better choice would be to put the cookies away. Your other guests will understand.
Thank you for welcoming this lady into your home.