The other thing that's getting a lot of attention for tendon problems these days, is eccentric strength training. So you can do calf lowers - skip most of the concentric phase of the exercise by stepping up onto your tiptoes and slowly lowering your heel to the floor. Keep a hand on a wall or bar for balance if you need it. Doing them barefoot (or at least without rigid shoes) will help strengthen your feet, too - which as I'm learning, weak feet can really be a culprit in Achilles problems.

Or you can do them seated on the floor with a strong Thera-Band around the balls of your feet - that really lets you do only the eccentric phase since you can let go of the tension on the band while you point your toes to get ready for the next rep.