There are lots of plus along the ground of newer buildings in both parks. Disneyland's older buildings don't have many of them, but DCA buildings are chock full of them. Just look for the covered outlet, sit down, plug in, and charge away. It's free!
At Downtown Disney, there are outlets in the planters along the main walkway. Look for a small green post with an outlet box on it-- they are usually on the ends and in the bushes a little, but definitely visible. It's nice because you can sit down on the cement bench/planter things (where everyone sits- not like on the floor or somewhere awkward) and listen to music or eat Haagen Dazs while charging your phone.