I always make a list of who I am buying souvenirs for. Sometimes, I even go as far as figuring out about how much I want to spend on each person. That way, when I go shopping, I can get something for everyone and buy within the budget I've set for each gift. It elimates over or under buying for friends and family. Another nice tip is to get a set of things that you can break up to make small gift bags (pen sets, collection of mint tins, candies, hair accessories, etc.).
For kids, it is always a good idea to get them Disney dollars. Decide on an amount ($20 or so) and tell them that is their treat money (snacks, souvenirs). They have fun spending it because it doesn't look like real money and you can set the rule that once their Disney dollars are gone, there is no more spending.
Definitely bring your own snacks and breakfast foods. I love bringing bread (or bagels), peanut butter, and jelly. It doesn't go bad and it is good for breakfast, lunch, or snack. Snack bars are good too (especially the hard, crunchy ones because they don't get smushed in your bag). Water bottles can be refilled throughout the day. You can also ask for a free cup of water at all counter service restaurants. All you see on the menu is bottled Dasani. Ask for a cup of water instead!!!