I can honestly say that with a family of 6, we spend almost zero money in the parks, other than food. Last time, we were on the dining plan, so we spent nothing. My souvenirs are my pictures. I don't buy the other stuff b/c I can buy it later at the
Disney store on eBay and then I don't have to cart it home!!!
For us, the best inexpensive souvenirs are pressed pennies. At 51cents each, you can't beat the price!!!! Plus, they are fun to make and collect!!!
As for saving with food, we think WDW meals are HUGE, so split them. If you get a TS meal with fries, IMO, the fries for one meal are enough to feed our entire family. One thing we learned by accident (and then discovered that it was a common "frugal tip") is to split a double cheeseburger! Neither of us like the white bread bun or tons of fries, so we ordered a double burger and each ate one meat patty and one half of the bun and half of the fries. PLENTY of food for us. We also have the kids split their meals at TS places b/c the portions are so huge. Sometimes we order kids' meals and sometimes adult meals ... depends on the place. It is easy to go back and order more if you are still hungry, but I can honestly say no one in our family leaves a WDW restaurant hungry.
We pretty much only drink ice water in the parks. Sometimes, our ds5 will get Sprite, but he is the only one who likes soda. The rest of us prefer plain tap water.
Also, you can ask the concession stands to refill water bottles (the kind with wide mouths we use for sports/school, etc., not regular Dasani-type bottles) with ice water for FREE. If you are going when it is hot, water is much better anyway!
In addition, you can take a small amt of snacks into the parks. I like to take "healthy" snacks like nuts, cheese, apple slices, etc. to balance the multitude of bad food they are going to eat during the day.