I just spoke to my Dr about this, she said that ancient cultures did not have the ability to completely rest while fasting. Gentle exercise is good to remind your body what muscles need to be saved. She recommended walking 1-2, miles a day. I would imagine bare foot on grass or the sand would be best.
I am also planning on a 40 day fast in September to disolate a thyroid tumor,
(along with a few other things) I was very concened with the resting vs active fasting. I want to be sure I am giving my body the best chance of healing.