Doing (forearm) planks will work triceps, upper back and some chest. Variations on this could help too, like putting out an arm and reaching for something OR both an and the opposite side's leg. I've had wrist tendonitis for the past week and a half, and this seems to be safe. If it's too easy, then you could try plopping a weight on your back while doing this--*if* you've done that before (if not, then it'd suck to injure something else). If you have access to a gym, a Smith machine does wonders. Like if you're comfortable doing Good Mornings, you could use the Smith machine; however, I have only one injured wrist; so, I used my good one to twist the bar off the rack. I'm not sure if this would be feasible if both your wrists are hurt. Also, not sure if your injury is like mine, but mine is throughout my hand, so I can't grip anything. However, if you have a wrist splint AND you're allowed to grip things (so, unlike my injury), then you can do a million things with cable machines or with at-home cables that attach to your door, like standing rows (narrow or wide), the sitting version of that, unilateral versions of that, chest press, ... you get the idea. ...I don't know how strong you are, so this might be too easy, but maybe you could do rear deltoid exercises on a bench without using any weight, by maybe sitting on the end of the bench, bent forward at a 45 degree angle to lift your arms up OR by lying on your stomach on the bench and slowly lifting your arms up. ...I expect my reply is unsatisfying, but hope it helps!!