Crochet has a different repetitive motion in your hands, and it can be a very nice break from knitting. I find that the two arts work your hands in different ways.
If you're into fiber arts in general, you might also try quilting, spinning and weaving (my aunt loves the fact she can get physical with her weaving - it is a great way to release frustration), bobbin lace (there are lace groups around the country who can help you get started http://www.internationaloldlacers.org/chapters.html
), cross-stitch, tatting.
As far as quilting goes, while machine quilting is faster, people made their quilts by hand for centuries. Hand quilting is much slower, but you could start with a wall- or lap-sized quilt so it would go faster.
Nordic Needle also has a bobbin lace kit that could get you started if you want to go that way: http://www.nordicneedle.com/PROD/Corrections/K1280.html