I took my DD when she was 10 or her birthday and I'm taking my DS in September for his 8th.
Just going with me and not the rest if the clan is the best part of the entire thing because we are a family of six. We get a few days alone to connect and share some special time together which is very rare.
For my DD, we had a fun dinner at Garden Grill and I ordered a special cake for her. They made a big fuss which was cute. I think we got a cupcake at each TS meal too. She had the pin and everyone made sure to say Happy Birthday.
When we got home, I made her a photo book of our vacation which she really loves.
When I go with my son in September, we have some fun but basic plans. On our last trip, he missed a lot of things that interested him because of one thing or another. My plan this trip is to get FP+ for his favorite rides and to hit his favorite restaurants. Whatever he wants to do. He'll get a button and a special cake and photo book too.
Just knowing that the trip is all for them, especially at 11 where she can really appreciate and understand the effort and cost it takes to put on a special vacation, might just be all you need (plus a cupcake at dinner!!).
Enjoy your trip! Keep us posted in what you ended up doing so we can get some good ideas.