Found a good stretching exercise for the neck:
Start with your head hanging down, chin touching
the chest. Just hang for awhile, don't force
the neck, just relax.
Then let your head go back (eyes looking up).
Again, don't force anything, just breath deep
and relax the neck muscles.
Then work the sides. Let your head hang to the
left towards your left shoulder.
Repeat for the other side. Let your head hang
for as long as necessary to stretch the muscles.

I've found the left side where I had neck
dissection surgery the toughest, so I follow up
with self massage for a few minutes.

There are several light exercises you can also
do to tone the neck muscles. One is to place
your hand on either the side or front of your
head and push against it.
Another is use a towel around your head and using
both hands, pull the towel to apply resistance.
Works for me
O-O O-O