Hello, Mariah!
What a great surprise for your sister's birthday!
There are PLENTY of places where you can find unique or quirky merchandise in Walt Disney World. Depending on your sister's tastes, you might find just what you're looking for at
Downtown Disney. You could
design her a T-shirt, find her an "edgy' accessory at TrenD, something sparkly at Something Silver or even stick with a "Sweet Sixteen" theme by loading up on sweets at Disney's Candy Cauldron. Of course, the world's largest
World of Disney Store is there, too, so you'll probably find plenty of unique merchandise tucked in and around that, too.
Consider the unique offerings in the
World Showcase Pavilions. You'll find all sorts of really unique gift ideas in each one of the Pavilions in the World Showcase; in fact, you could make it a point to tour the shops, taking note of something in particular your sister really seems to love, then sneak off at some point to pick it up without her knowledge. You can then have it delivered to the park entrance, so she'll never even see you with it when you rejoin your group. If you are staying at a WDW resort, you can even have the package delivered to your resort at no additional charge.
Be sure to pick up a birthday button at Guest Relations, too, so that everyone who sees your sister that day will wish her a very happy birthday. (Dozens and dozens of birthday wishes are sure to bring a smile to her face!)
What a fun trip you're planning! I wish your sister a very happy 16th birthday!
Send more questions as you think of them; I'd love to help!