I used a handheld trackball (which I held behind my back) for a couple of years when I was unable to use a mouse for longer than 10 seconds.
But I would not call it comfortable and it took a lot of time to get used to.
Amazon.com: Image Wireless USB Handheld Finger Trackball Mouse with Laser Pointer: Computers & Accessories

Tablets are evil as far as ergonomics goes. I am trying very hard to avoid them.