I've done several very frugal Disney trips in the past, and now I'm in the middle of planning our all-out deluxe trip of a life time trip for my extended family. Some of my favorite tips:
For food:
-if you take a towncar from the airport to your hotel, most include a stop at the grocery store. We use this to stock up on fresh Florida oranges, cheese and crackers, bagels and cream cheese, and milk.
-if you don't have a grocery stop, lots of people arrange for a grocery delivery from garden grocer or another online grocery shopping website. I've not used them myself, but lots of people do this.
-another option is to pack a suitcase or send a package (flat rate USPS boxes are about $9) to your hotel with dry goods. Pack cereal, bagels, chips, crackers etc, and then buy milk at the hotel (most have a small marketplace type shop with half gallons of milk, juice, fruit, etc).
-if you have perishables and there is no fridge in your room (aka at value resorts) you can do a couple of things. We would just put the items into the sink and fill it up with ice each morning. A small cooler packed into your luggage can be used the same way. If you want a fridge at Pop or All Stars look at the resort boards for a fridge swap. Groups of Dis people sometimes go in on purchasing a small fridge, costing about $10 a person, and the fridge is passed from one person to another. After all of the people that paid for the fridge have had a chance to use it, the fridges usually stay in circulation free of charge to other Dis people. All you have to do is sign up, claim it when you check in, and then leave the information of the next arriving guest on the box when you check out. More detailed info is on the boards, but it's something that's available.
-Bring a piece of fruit and a granola bar with you to the park, you never know when someone will get hungry, and do you really want to pay $3.95 for an apple?
-Purchase crystal light and lipton single serve drink mixes. Ask for a cup of ice water at a Counter Service, and you've got an instant drink. I personally prefer water, so I keep a big 32oz Nalgene bottle with me at all times. I purchased one of the Disney drink lanyards in Epcot once and have used it for years to carry my water bottle.
-I've switched from carrying my water bottle to a
hydration pack, which is like a small backpack that has a reservoir for water and a tube that comes out around the shoulder. I've found that having the bite valve right near my face constantly reminds me to keep drinking water, which is so important. I like to fill it with ice from the hotel in the morning, and then add water from the tap until it's full. The reservoir is right up along the back part of the pack, and so putting it on when the temps are high automatically cools you down. Fill it with ice water from the counter service, or fill at the fountains. Add drink mix at your own peril - the artificial colors will dye the plastic and if there's sugar in the mix it can grow yeast and bacteria inside. Best to stick with water.
-If you're going to eat breakfast at the hotel, and then budget for one counter service and one table service, do the table service for lunch and the counter for dinner. Most of the Table services have a nearly identical menu for lunch and dinner, but lunch is significantly less expensive. This would also be a great mid-day break (see below in my tourining tips) without leaving the park. You'll be inside, leisurely enjoying your meal in the air conditioning while the sun is high overhead and most people start to run out of steam. Have a nice lunch and then maybe get a counter service meal to eat while you pick out seats for the spectromagic. If you're not terribly hungry, get two meals for the three of you to split.
-The buffet meals are a good value, and help a lot for budgeting purposes. Again, a breakfast or lunch buffet will be $10-20 cheaper than the dinner buffet at the same location, so for three people that will save $30-60 at one meal. My favorites are breakfast at 'Ohana, and lunch at Crystal Palace.
For touring:
-I mentioned it above, but I LOVE my hydration pack. It's just big enough to fit my wallet, sunscreen, bandanna, camera and a few other necessities, but not so big that you become the family's pack mule for the day. I have seen them at Costco and Sam's for about $20, and REI has child-sized ones. If you can afford to have one for everyone, I'd recommend it, as everyone is then responsible for carrying their own items and no one person gets stuck schlepping everything around. They're small enough that you can stow them on the rides without any hassle. I take mine off, tuck the bite valve into a zippered section (don't want it touching the floor of the ride vehicle!) and put it on the floor. For faster rides I loop it around my ankle, and for the really active rides there is usually a pouch for you to tuck your belongings into, these fit perfectly.
-If you want to approach your trip like a Disney pro, you'll want to arrive when the park opens, stay until 11 or so, and then go back to your resort to rest. It'll get you out of the sun during the peak, everyone can take a nap, eat some lunch, watch TV or a DVD, swim in the resort pool, whatever. Then you can go back to the parks refreshed and ready for the rest of the night. This will especially help if you plan to stay for fireworks etc, which often are past most kids (and my!) bedtime. If you try and go commando all day everyone will burn out. Do this every day and you're setting yourself up for a miserable trip.
For souvenirs:
-Try to go with a theme. One year I bought only scrapbook items. When I got home I had great papers and stickers that you won't find at Hobby Lobby or even Archivers, and I had both a fantastic souvenir and supplies for one of my hobbies. Think outside of the box, I collected things like business cards for the restaurants we visited, and pressed pennies from around the park to decorate my pages, you don't have to get it all at the scrapbooking store at Downtown Disney. If you're into golf, then buy a few nice polo shirts and maybe some golf balls. If you like to listen to music buy a few CDs of park music to toss onto your iPod and remind you of your trip. Whatever you're into, buy items that you'll actually use in your life. As nice as those snowglobes and stuffed animals are, will you actually use them once you get home, or will they just sit on a shelf? (Also, snowglobes cannot be carried on to airplanes due to the new 3oz liquid restrictions. Be sure to ship it home from the store if you buy one, or have it carefully packed so you can put it into your checked luggage.)
-If you're buying gifts for friends and family at home, be creative. I once bought Disney pins for all of my family members as a gift, but at nearly $8 each, it got pricey. If you do the dining plan use some of your snack credits to buy fudge or candy to bring home. Pretty much anything with a Disney logo on it is a good souvenir, I especially like getting postcards for people. They are different at all of the parks, you can find a Star Wars one for the brother in law, a princess one for a niece, there's something for everyone and they are relatively cheap. Bring postcard stamps from home and mail them in the Magic Kingdom, they'll have a beautiful Disney post mark.
-I agree that you should decide what to spend and then find what to get, as opposed to figuring out how much you'll need. Tell your daughter you will buy her one "big" thing and she has to use her own money for anything else, or set a limit of $50 and let her choose one larger item or lots of small things. As long as the ground rules are laid out, it shouldn't be a problem. If on day one she decides she wants the giant stuffed mickey from Main Street, remind her that it will still be there on your last day, so maybe she will want to keep her eyes open for different things she wants to consider, and then make purchases on the last day. Use a small notebook to jot down the location and price of each item, and on the last night look at the list and make some decisions. If you have a digital camera, take pictures of each item so that she'll have a visual to help make her decision.
Remember that extras are just that. As long as you have park admission and some money for food, you're going to have a good trip. I mean, you're in Disney World, how can you not?! Decide what's important to you, and make those things priorities. Decide how much money you want to spend on your vacation, and then plan to fit that budget.