Fantasmic! (always has an exclamation mark) is a great show. It's not really fireworks, but there are some pyrotechnic effects. There are live actors doing different scenes, water projections, and lighting effects. There is not much dialogue or anything, but the musical score (which is wonderful) brings it all together.
While Dining Package guests do have a reserved section, it is a small portion of the theater. If you want to sit close you will still have to get there pretty early. If you don't mind sitting in the back, then you are guaranteed a seat in the center-most area (best viewing). Some people like to sit toward the back since you can get out ahead of the crowd, plus the first few rows may get wet from the water screens if the wind is blowing that way (it's really a light misting, but after 20 minutes or so you may find yourself rather wet).
Another option is to get a Fastpass for it. They are usually easily available as a fourth or fifth one of the day (don't waste one of your main 3 on it). This will give you the ability to choose form either side of the theater, including the other center-most section (if you are there early enough). While Fastpass does not really let you show up any later, it will let you pick to sit on the entire FP side of the theater or the standby side should you prefer. Very close to showtime they will start letting people sit anywhere too, so the FP side can fill up.
I know getting there early is a pain (and the pre-show they sometimes do is pretty grating), but Fantasmic! is one of my favorite shows and well worth seeing.