Water - free at many locations within the park including CS restaurants (cup of ice water / not bottled)
Go with the flow - something will probably happen to alter your plans (for example, RnR has been down the last two days) - don't let this ruin your day or trip - thoroughly enjoy what you get to do and don't dwell on what you miss or what isn't quite as magical as you expected
Pictures - make sure to stop and take them! (but not so often that you irritate everyone with you)
Foot wear - make sure to have an alternative pair of flip flops or other sandal to take to the pool. Not fun walking around with wet athletic shoes because you forgot sandals / flip flops (this usually isn't an issue for females but have forgotten for DH and DS) - A cheap pair from the dollar store can be a life saver!
Zip locs - if you plan on going on water rides - zip locs can come in handy for items you don't want to get wet
Water rides - if you are going on water rides and don't like to spend your day (more specifically when cool but even when hot) with wet shoes, undies, etc. - wait to go on them later in the day before heading back to the resort
Snacks - especially if you have kids - make sure to pack snacks into the parks so you can avoid the over hungry crankiness and not be spending a ton of money on snacks all day. My younger DS was a "grazer" when he was little and while we always had a special snack break from the park it was important to also have other snacks like granola bars, goldfish, etc. packed into a bag. (but avoid chocolate if it is hot or you will have a mess!)
Sun tan lotion - do not forget this or underestimate the need for it (even on cloudy days) - AND also lip balm with SPF (I carry a Blistex stick in my pocket or bag)
Gum - if you or others with you are gum chewers - bring it with you - don't believe you can buy at WDW
Rope drop - even if you can't every morning, I highly suggest you do rope drop at least a couple of times as this is a great way to get a lot of rides done
Souvenir budget - I always like to go with an approx. budget for the kids (and myself too) on how much we will spend on souvenirs so we don't get carried away. I use Disney gift cards too which helps since sometimes the Magic Band somehow adds up rather quickly when you just have to tap and enter your pin in and away you go.
Hope this helps - have a wonderful trip!