There are different viewing areas for fireworks all around the Magic Kingdom and you don't want to miss them! The best one for me is on Main Street by Casey's Corner. You can see the fireworks over the castle.
Once you've done that, the next night, try a trip to the Polynesian hotel. You can eat at Ohanas and they'll dim the lights, play the fireworks music in and you can watch them out of the window.
Even better, you could have a romantic walk on the beach and watch from there
We have loads of little traditions at Disney, one of which is to watch the fireworks from the beach of the polynesian on our first and last nights