I've read more than a few lists on situation specific items to pack when vacationing to a theme park. I haven't been to Disney in a number of years, I thought I was prepared. But the lists I read left off a few key things that would have saved me a few headaches.
I'm not going to list every single item you should bring (I think comfortable shoes are a given) but this is what I brought and some things I wish I would have brought.
Backpack: What are you going to put all your gear in? A backpack is your safest bet (steer clear of the fanny pack). Sure, comfortable shoes are a given but some might overlook a comfortable pack. I went with the Incase Nylon Campus Pack.
This particular pack has a faux-fur lined pocket for up to a 15-inch laptop. Pretty standard compartments inside and one large outer pocket. The real selling point, as a park backpack, is how light it is and how comfortable it was to wear all day. I wore this for around 12 hours a day and never felt the strain that normal backpacks would give me. My girlfriend had a pack that she thought was more stylish but it really did a number on her throughout the day. In addition to having great shoulder padding there is a lot of breathability to the material. This design comes in a number of colors and while it is usually priced around $60 on Amazon you can sometimes find it for $35-$40.
Camera: Arguably one of the most important things to pack on a theme park trip, it is also the most divisive category on this list. I don't take a lot of pictures in my day to day life and for years my only camera was my phone. When planning our trip I knew I'd need a dedicated camera, if only to save on my phones battery life.
I went with the Canon PowerShot ELPH 320. I picked it up at Amazon for a reasonable price of $140 but it has been as low as $120. This camera did everything I needed. Took some great looking pictures (you can see all my photos taken with this camera here) and impressive video (which I used to make this). This camera was very small and light, which was essential because I held it on my wrist with the included strap the entire time. You might want to carry around a DSLR and take some even better photos. Find something in your price range that will meet all your needs.
Camera Battery/SD card: So you settled on a camera. Do yourself a favor and bring along an extra (fully charged) battery. I foolishly overlooked a backup battery and was milking the last few seconds out of the battery by the end of every day.
You'll also need a SD card. I took around 150-200 pictures a day and a lot of video and never pushed the limits of my 16GB SD card. Depending on your habits you might want to pick up two.
Card Reader for iPad: I only have a desktop computer so I brought my iPad mini and my iPhone with me. My camera had a wifi feature that allowed me to take pictures off the SD card and edit them on my iPad every night.
Only problem was the camera's interface was slow and horribly clunky. If I had a camera connection kit it would have made things a lot easier. If you're not bringing a laptop I suggest having one of these handy for your iDevices.
Battery Backup: Another item I foolishly did not think of packing was a battery pack for my phone. These little battery packs are becoming more popular lately. You should look into how many charges you can get out of it in relation to price.
The best reviewed one I've found is the Jackery Bar. That version seems like a good bet for most people, there are smaller and larger version depending on your power needs. For $30 your phone and your camera should be covered for a normal day.
Hand Sanitizer: I seldom use hand sanitizer in my day to day life but you are going to need it in a theme park. Depending on your level of germaphobia, pack a bottle or one of those tiny ones to clip onto your pack.
Ponchos: On our last trip we got very lucky, it only rained for a few hours one day but having ponchos ready to go saved our behinds. You can get some on Amazon for a few dollars. Buying them in the park will be many times higher in price. Plus you have to get in line with all the other dummies who forgot to pack ponchos beforehand. However you get them, just make sure you have a few for each member of the party.
Plastic bags: Grab a box of ziploc backs throw them in your bag. They give me piece of mind when going on any water ride with my phone or camera. My girlfriend didn't believe, and that her purse was just as good as my ziploc when riding Roman Rapids at Busch Gardens. Her phone did not survive.
So those are some of the items I see as essential and some easily overlooked. Have something to add? Email me any suggestions and down the road there will be an updated list.