I haven't had the neck pain but have read about it, and I think Jensip is right about seeing a PT for specific stretches.
More generally, I would recommend yoga, massage and easy walking. Those are all investments with a payoff that will be better than being inactive.
Best wishes.

Randy, Stage 3 tonsil SCC, node involvement, HPV-positive. 7 sessions of Taxol-Carboplatin, 35 rads finished 9/8/10. Post-TX PET and CT 12/1 showed "no metabolic evidence of residual or recurrent disease." Yay!