Welcome to planDisney, Erik! 
Oh I am SO EXCITED that you trusted us with this question! It’s wonderful to hear that you’re planning a visit for your daughter to Disney World for her very first time! A 9th birthday celebrated at The Most Magical Place on Earth is going to be just that — THE MOST MAGICAL! 
While I do not know your daughter personally, I can speak from the fact that I was once a 9-year-old girl at Disney World too — and I can let you know what I would have enjoyed the most for my birthday! 
My vote would be to surprise her with a 
MagicBand+ for Christmas…and maybe throw in a couple of 
Disney Trading Pins and a lanyard so she can trade pins while at Disney World! The reason I vote for this is because I LOVED being part of the planning process with my parents. I adored helping my mom plan our outfits, helping my dad think through the restaurants we were going to visit, and I even loved packing! There is so much you can surprise her with once at the theme parks too! 
A special dining reservation she doesn’t know about (like at 
Cinderella’s Royal Table) or a 
Fireworks Dessert Party could be incredible to keep a surprise so she can still get that feeling of pixie dust while once on vacation! 
I also want you to be sure to get her complimentary “Happy Birthday” button at any of the gift shops so she can wear it during your trip! You never know what extra magic will follow because of that little button! 
If you ever have additional questions, Erik, I hope you’ll visit again! It was so lovely to be entrusted with this, and I hope your daughter’s birthday is one she’ll never forget! 
Have a great, big, beautiful tomorrow!
Allyn