I like to bring a grab-n-go breakfast and snacks: granola bars, cereal bars, poptarts (only because it is vacation), mini cereal boxes (for some reason, these are much more delicious than a regular big box) bagels and peanut butter.
For snacks: pretzels, peanut butter crackers, chips ahoy, fruit leather, cheezits, ritz crackers, 100 calorie chips. I also save the goldfish included w/the kids meals for later...they come in handy!
I read a great tip on here to pack crushable snacks in the clear shoe boxes to protect them in your luggage. You can also ship them to yourself at your resort if your airline had luggage restrictions (or your car is bursting!!).
We have avoided many a meltdown by being able to whip out a snack w/o having to wait in line. We usually order drinks,fruit and other perishables from gardengrocer.com so we can have a ready supply of juice boxes on hand as well. We carry a small cooler bag into the parks w/drinks and snacks and it is the handiest thing we bring to the parks!