I fear for this country when this statement becomes a marketing tool:
Great for travel or staying on the couch as long as possible
We got Mophies for our phones before our last trip and they worked well...
In addition, halfway through our trip I figured out this tip - Instead of checking our schedule on our phones all day, I would take a picture of our schedule and made it the lock screen picture on my phone. Saved a bit of the battery life...
One of the power packs can take care of your charging needs. There are charging stations in the MK too. One in new Fantasyland and one in the gift shop at the end of Space Mountain. I'm sure there are others but those come to mind. On my iPhone I use a Mophie case with a built in battery. It just about doubles the battery life. YMMV.
I know it's easier said than done, as I'm always attached to my phone, but try and spend time without looking at it every two seconds and enjoy the fact that you are at Walt Disney World!
Here's a great article for upping your battery life with the iPhone 5.
http://www.overthought.org/blog/2014/the-ultimate-guide-to-solving-ios-battery-drain
One thing he doesn't mention is (in Settings) go to Wallpapers and Brightness. Turn off Auto Brightness with the toggle and set it to a dimmer setting (you can always adjust this)
Also, click "Choose a new Wallpaper" and select a "Still" Wallpaper instead of "Dynamic" Dynamic wallpapers move, thus using battery!
There are lists more tips, but I think a Google search might work just as well. I would recheck your battery life after adjusting your settings, THEN decide if an external battery is right for you!