As someone who's spouse is currently in their 3rd year of rehabilitation after TMJ (tempo-mandibular joint) reconstruction, let me warn you against any kind of chewing activity that involves either high direct pressures (ice, hard things that "crack", anything that resists biting down very much), or strong lateral forces (bagels, pizza crust).
We're about $60K in to her treatment. She's had the meniscus of the joints on both sides of her jaw surgically repaired and now is undergoing orthodontia to permit her jaw to safely re-align.
This after a year of excruciating pain (the TMJ was bone to bone contact), and a year of painful muscular rehab. Unless you are a maxillofacial surgeon or perhaps a particular specialty of orthodontia you are probably unaware of just how many muscles in the head have to re-learn how to work after TMJ problems.
The "straw that broke the camel's back" in her case?
A pistachio nut.