I’ve just been diagnosed with tendonitis. How can I keep my arm strong while I heal?

I played a little too much tennis and developed tendonitis in my right wrist. The doctor has put me in a rmovable brace to immobilize the area for a wew week, at which time I’ll get physical therapy. However, having my wrist immobilized for a such a long timewill deteriorate the muscle. I’m wondering how I can keep my arm fit without worsening the tendonitis.