I have a non-trivial case of tendonitis in my left shoulder and my doctor’s advice for it is mild stretching, Aleve for the inflammation, and an ice pack to deal with swelling. The ice pack I’ve been using: A bag of frozen peas, because it’s conveniently sized for my shoulder and because the frozen peas are cold without melting on me and/or giving me frostbite.
With that said, the regular thawing and refreezing of the peas in question are likely reducing their usefulness as actual foodstuffs, so we’ve marked the particular bag of peas I’m using as “shoulder peas” to make sure no one opens up the bag and tries to, you know, eat the things. Please never eat the shoulder peas, folks. I know where they’ve been. On my shoulder. Over and over again.
Also, tendonitis sucks, and I don’t recommend it to you. Especially if you’ve gotten it the way I have, which is to sleep on your arm wrong. Aging sucks, y’all.