I personally would buy my sister this
Seagate Slim. My sister is dumb as a sack of potatoes when it comes to her Mac Book so in my eyes simple is more stress free for me.
This hard drive comes with a Dashboard, iPhone app, is pre-formatted for MAC, and is USB 3.0(which is compatible with USB 2.0.) In my opinion it's a better quality drive than others in it's price range.
However, I would recommend if you have the time or ability to save up more I would probably buy a LaCie drive over anything else.
Again I would only buy this for my sister. If I were purchasing for myself I would probably extend my search past the "ease of use" hard drives.