iPad app where possible draw, save for reuse and drop 'objects' on canvas for prototyping and to make mockup
The best app that comes to mind is Sketchbook Pro for the iPad. It's like a simplified version of Photoshop where it has brushes and a lot of features that are at the mercy of your fingertips. I have it, and it's great for prototyping / making concept art real quick.

http://itunes.apple.com/us/app/sketchbook-pro-for-ipad/id364253478?mt=8

I don't know what you mean by saving and dropping templates onto a canvas, but hopefully this link will steer you in the right direction.