Let's see. You asked a lot, and you've gotten a lot of responses - I hope I add something new to the mix!
First of all, the dining plan. It pays for some people, it doesn't for others. Totally depends on how your family eats and how you like to vacation. There's something to be said for having everything paid for in advance, no worries, no need to look at the menu prices and debate whether a dish is worth the splurge. We've done the deluxe dining plan a few times precisely because of that, and because we like to eat good table service meals, not so much quick service. (There's only so much counter-service food I can handle before my stomach becomes a very unpleasant traveling companion.) So we've done the deluxe dining plan, eaten mostly at 2-credit "signature" restaurants at night, mixing in the occasional character breakfast, and otherwise had lighter meals/snacks to keep us fueled for the parks. And we bring home sacks of Chip & Dale pretzels and other Disney goodies that make for nice treats once the trip is over, because really, the amount of snacks you get on that plan is too much! That said, we aren't doing the dining plan on our next trip, as we're planning a day or two at Universal in the middle, and we obviously aren't willing to pay for the days when we'll be eating elsewhere. You have to consider how you like to eat/schedule your day, look at the prices in the restaurants you most want, and figure out where the balance tips for you. What works for one doesn't necessarily work for another. YMMV.
In September, you're going to be overlapping with my personal favorite Disney event - the Food & Wine Festival in EPCOT. Talk about good eats! It gets crazy in the World Showcase on weekends during the event, so I'd avoid EPCOT on weekends/Friday nights on a September trip. But it's so fun to wander around the World Showcase and do a meal made up of tastes as you go. As long as you don't mind eating standing up - if you can snag a bench to sit on, more power to you!
We did the MNSSHP in fall 2014 with my then-5 1/2 year old son. He was actually not a fan - he's a sensitive kid, and even the "not-so-scary" was a bit unsettling for him. Poor kid. It probably didn't help that we were there for a sold out party right before Halloween and some of the people (adults who should know better) were pushy and rude - like one woman who nearly knocked him over as he stood by the rope for the parade. She shoved him out of her way and stood in front of him, blocking his view as the parade went by. He was devastated. And frankly, he was 40 inches tall. She could have seen just fine from behind. (Ugh. Some people.) In any case, his lack of enjoyment made it hard for me to enjoy the Halloween party, but looking at it from an outside perspective, it was pretty darn cool. That might be worth a late night for you, your wife, and your daughter, if you're all into Halloween!
Other things we've done that have been TOTALLY worth it - character meals. Our favorite is Tusker House in Animal Kingdom. Tasty food, with traditional American stuff mixed with some unique African-inspired flavors if you're up for something new and unusual. And the character interaction we've had there has been terrific. We also had great character interaction at Cinderella's Royal Table - we booked that when my son was starry-eyed about princesses, but by the time we arrived at Disney, he was convinced that princesses were only for girls. (He was 3 1/2 on that trip.) So he refused to so much as make eye contact, and the princesses were so sweet and all tried SO hard to get a little attention and affection from our "reluctant prince." It was a riot. We've also enjoyed dinner at 1900 Park Faire (Cinderella, her prince, stepmother, and stepsisters). Hilarious interactions. We aren't fans of Crystal Palace in Magic Kingdom, though some swear by it. We weren't impressed by the food, and our character interactions with Pooh & co were underwhelming.
If you think your little girl will enjoy something pirate-y, there are two awesome pirate-themed activities I can recommend. The first is the Pirates & Pals fireworks cruise - it's for the whole family, you get a little "mocktail" hour with snacks and drinks while Captain Hook and Mr Smee work the room and do photo-ops with people. Then everyone troops on board one of Disney's boats (pontoon boat, I think?) for a "cruise" along the lake to a spot where you can see the Magic Kingdom fireworks. Along the way, you're entertained by a rather funny pirate named Patch, who gets the crowd interacting and laughing. Then the boat stops and cuts engine for the fireworks. You're far enough away from the fireworks that the booms aren't enormous if your kids have sensitive hearing, and you can see the whole thing - a totally different perspective than being right under it at the Magic Kingdom itself. The boat gets the music piped in, so you hear the whole thing. And there's a special surprise character waiting for your boat to return back at the dock. My son was 3 1/2 when we did this, I think? And he was in heaven the whole time. I have photos of him dancing with Smee and hanging with the surprise character. So fun.
The other pirate thing, which is kids-only, is a pirate treasure hunt. I don't remember the exact name, but it's run certain mornings during the week from 4 different marinas across Disney World. We did the one that left from the marina at the Yacht Club. We did this on our trip when our son was 5 1/2 and it was AWESOME! We brought our son over, saw him snapped into a pirate-themed lifevest, then waved as the boat departed with a handful of kids and a few supervisors along. The kids went on a pirate treasure hunt that took about 2 hours, if I recall correctly. So my husband and I had 2 hours to ourselves to just sit and relax and chat without a small voice piping up and interrupting! Our little guy came back full of stories of the pirate they chased and how much fun they had, and at the end, all the kids share the "treasure" that they found - silly little toys and things, but it really was treasure to him at that age! So fun. We're planning to book that again on our upcoming trip.
There's probably more, but I have to run. Check the Disney website for some ideas...
OH! And one more. Buy the Memory Maker package if you're a family that appreciates photos at all. Everyone gets in the picture. It's the only time I'm in vacation photos - when we're at Disney World. And we've had some really awesome photographers who have captured some really special memories...
If I think of anything else, I'll pop back later to add.......
