Hi Amy,
What a fun way to celebrate your daughter's 13th birthday! You cannot purchase the Disney Dining Plan unless you are staying in a Walt Disney World Resort hotel; additional discounts can only be acquired if you are a Florida Resident, an Annual/Seasonal Passholder or if you are a Disney Vacation Club owner, in which case I would recommend looking into Tables in Wonderland (www.tablesinwonderland.com).
As for
Planet Hollywood, it is an independently-owned restaurant located in Downtown Disney. I would recommend calling the restaurant directly to see if they offer any special desserts for birthday celebrations. If your heart isn't set on a full-sized cake, I highly recommend spending your weekend scouting out the many cupcakes available throughout the Walt Disney World Resort! Not only do the theme parks have delicious cupcakes available, but many resorts do also. My current favorite is the Frozen Fever
Anna's Happy Birthday Cupcake-it's delicious!!
And one more thing you might consider is attending my favorite special celebration of the year,
Mickey's Not-So-Scary Halloween Party! It's not just for little kids, I promise! It's a fun-filled night full of costumes, a great parade and lots of trick-or-treating!
I hope you and your daughter have a wonderful time celebrating!!
Best Wishes,
Amy B.