I love any parents who want to spend a birthday at Walt Disney World, Lexie! What a fun way for your mom to celebrate. As for fun and nice, I can think of a few. Here are my suggestions:
Teppan Edo - Located in Epcot's World Showcase, this hibachi style restaurant offers great ambiance with sophisticated decor and fresh, delicious food cooked right in front of your eyes (which satisfies the fun factor, don't you think?)
Jiko - As one of Disney's signature restaurants, Jiko offers delicious meals and superb service. The fun here lies in what you do before or after the meal. I'd encourage you to take a walk out to the savanna to view the animals. In the past, our Jiko servers have always offered us night vision goggles after our meal to get a different perspective of the animals after dark. Fun, right?
My last suggestion would be
California Grill. Another signature restaurant, California Grill offers not only amazing food, but also a sweeping view of the Magic Kingdom. Schedule a reservation for an hour before the Magic Kingdom fireworks and your mom's birthday is sure to go out with a bang!
I'd be more than happy to celebrate my own birthday in any of these restaurants; hopefully your mom will, too!