D.W.
I had a customer a few years ago who had the same spinal cord injury as Christoper Reeve - only, he was able to walk after a few weeks and resume normal life.
We got him a boredom basket with all kinds of magazines and other things to help take his time away. A magazine can only be read so many times, as well as movies. Perhaps a deck of cards to help entertain him. Depending on how much you want to spend, you could even invest in one of the small personal lap tops (usually ~$300) so he could have internet access, watch shows on Hulu, etc. That's on the high-end, of course, and would be way out of our budget.
My husband loves the DuPont registry (high end car magazine) and can read that for days. Does he have any favorite authors? You could go to the library and get some books if you don't want to purchase them.
The bell is funny - I went through 5 months of chemo, and the stress of being the primary caregiver can be overwhelming - while a bell may be a good idea initially, it may backfire in the end if they don't have the right sense of humor (like others mentioned).
I'd also try to put something together for her to help take the stress away. If you're close and can even offer to come over and help with laundry, dishes, dusting, whatever - even be there to tend to him should she need to get out and get groceries, etc.
In the end, it's the little things that really make the biggest difference. It's really nice that you're wanting to help him out.