I was just there back in May and did all of the parks. The cost of bottled water was quite reasonable, surprisingly. It was only $2. We thought it would be near $4 a bottle. I do have to say, almost any restaurant you visit within the parks are quite exceptional for "amusement park" food. I would suggest Epcot for most of your eating out. Pick a country and have at it. We went to the German buffet twice over the week. I forget the name of it right now, but for ~$21, I was shocked at the quality of the meal/buffet. Oh, and it only cost $10 for a liter of beer(I'm used to paying roughly $5 locally for just a pint!). The main Moroccan restaurant was also quite good(not a buffet). There's only a handful of buffets in all of the parks combined.
The House of Terror is at MGM and is definitely worth waiting for. Always make use of the Fast Passes whenever possible. I believe just about every ride incorporates it and is worth not waiting in a huge line. Just go up to the Fastpass booth, insert your entry ticket, and it will print out a fastpass which will tell you to come back in an hour or so. There are two lines -- one for regular walkups, and the fastpass line. If the timeframe on your fastpass ticket is right now, just walk in that line and you'll almost end up right at the front of the line where you get on. USE THIS BENEFIT ALL DAY LONG. Definitely use it for 'Soaring' at Epcot. This was consistently the longest line of all the parks we went to. In May it wasn't terribly busy. Soaring was always around 60-75 minutes long. Not sure during the holiday period you'll be there - might be much worse.
Anything else you want to know, just ask.