Hi Ashley!
Happy Anniversary! I don't think there's anywhere better to celebrate than at Walt Disney World.
I think a good place to have an anniversary dinner that fits all your criteria would be Epcot. There are so many fabulous restaurants in the World Showcase and I'm sure that one of them would fit what you're looking for. Here are some of my recommendations to get you started:
San Angel Inn - This restaurant is located in the Mexican pavilion and is a nice mix of romantic and fun. The setting is under a starry sky with Mayan ruins and a volcano in the background. You can also watch the boats from the Gran Fiesta Tour ride float by as you eat.
Restaurant Marrakesh - Located in Morocco, this restaurant features different and delicious food with belly dancers for entertainment. If you're on a budget there are sampler platters that include a variety of items.
Les Chefs de France - This restaurant located in the France pavilion is one of my favorites. The food is wonderful and nothing says romance like spending an evening in France! There is a prix fixe three course menu available at lunch and dinner which offers a great value. I feel that this would be a wonderful choice.
Coral Reef - This restaurant is located in Future World in the Seas with Nemo and Friends pavilion. This would be a great choice for a romantic dinner because one entire wall of the restaurant is a huge tank with hundreds of fish and various sea life. It is a beautiful and very relaxing environment.
I hope this gave you some ideas! Enjoy your upcoming celebration!