Here’s one thing to do with an older iPad if you recently replaced it with the new Retina model:
Get Air Display on the iOS App Store ($10) and use that older iPad as a second display when you’re working away from your regular desk.
In my office, I’m used to two good-sized monitors, and I typically have my editing environment on one side and a browser or documentation on the other. My laptop is a 13″ MacBook Air, and the screen is simply not large enough for that arrangement. With Air Display, it works great.