When we travel to WDW (always by plane, although I wouldn't mind driving once) with our DD's (DD5 & DD2), we bring new and older items for them.
DD5 is old enough now to keep watch of her 15 in. Ariel rolling backpack. I pack it with a blank notepad, crayons, pencils, pens, coloring books, reading books, and sticker books. This provides for both daughters, but the oldest is excited to be "in charge" of it all.
My husband and I love our iPads and iPods, so we bring one of each (which is loaded with more kids apps than adult-geared ones; plus we have lots of movies downloaded onto it). This allows for entertainment, for both kids and adults, in the airport terminal, on the plane, on ME, and on occasion, in line for a ride (very rare) or while waiting for a parade to start. The iPad and iPod are so much easier to transport than our DD5's Leapster Explorer, plus games.
As for snacks, I bring one grocery bag full of little snack bags, that I pack in one of our carry-ons. I buy all travel-sized products, such as: 100 calorie pack cookies, Goldfish crackers, Gerbers freeze-dried yogurt bites, Nutella with breadsticks travel packs (found at Walmart in one of the check-out lines), Ritz Crackers, cereal/granola bars, Dum Dums & gum (only given on the plane to help ease the ear-popping) and Rice Krispie treats. In my checked luggage, I pack Gerber yogurt & Snack Pack puddings (which doesn't need refrigerated until opened). We don't go through all of this, but it gives my kids (including husband), options during the entire trip!
Oh, for Christmas, I bought our little family Bobbles, a refillable water bottle that has a filter built into the mouthpiece. I'm hoping that these cut down on the need to buy bottled water (after all, I might as well take use of the FREE things Disney offers, like water fountains).
Lastly, as for clothing, I shop at a couple of different stores. Old Navy, Walmart and Target all have Disney logo apparel. We also live an hour away from a Disney Outlet store, so I pick up $3-5 t-shirts for the girls. I also pick up one or two "surprise" gifts that will be given sometime during the trip. However, we still give each daughter a Disney gift card with $X on it that they can use to buy anything they want. It teaches the oldest to be responsible, but not so much the youngest.
I have to echo what one poster said, bring your own ponchos! I buy one for each family member, plus one more adult-sized one. I use the extra one to cover the stroller. At the end of the trip, if they're still in good condition, I'll bring them home...if not, I'm out $5, compared to $35 ($7 ea. bought in WDW).
Enjoy your trip and the Flower & Garden Festival!:wave: