I have been doing Water Fasting for the past two years. I have had several longer fasts (14 days and a 21 day). When I first started fasting I tried to continue with Crossfit. It didn't go well. I was very weak, but I triumped through. Then I started reading more about the healing of the muscle mass, and this kind of strenuous exercise is actually depleting certain amino acids and other nutrients. I decided to not do Crossfit while fasting.
On my 21 day Water Fast I was doing light walking or swimming (3-4x/week). I found this very helpful for me to minimize some of my toxic systems while fasting. I also do some Yoga to help my flexibility. I think because I do Crossfit as part of my normal life process my muscles take a beating. The light exercise (3-4 days/week) for some reasons helps the muscle aches while I fast. Swimming is by far the best but I am not a fan of the chlorine/bromine toxins in the pool. So I go to walking (hopefully soaking up as much sun as possible) more than swimming in a pool.
Personally I like to get some sweating in while fasting. I make sure I am not dehydrated so I start after the first three days. The sweating helps me reduce some of my toxic symptoms.
I always dry fast for the first 3 days and then start on sipping warm water throughout the day. Once I know I am hydrated I will then introduce light exercise.
On my longer fasts if I find I go through a healing crisis then I simply sleep. I do nothing when I have lows.
Most of my fasts range 5-10 days. I have only done one 21 day fast, and I did walk and swim in the lake throughout. It was during the summer, so to me laying in the sun and swimming was refreshing. I like to do most of my longer fasts in the summer because I am a fan of sun, and feel the sun really does me good energy wise. I continue with my gardening while I fast in the summer. Again it lets me move and get the sun in while also growing my food.
I agree with not over doing it at all when you fast. I will let you know I am a single mom of four, so in all reality I have little downtime. I am full of energy, so a fast is my refreshing break. I find it is something that has dramatically improved my life. I look younger, have really reduced my cellulite, healed my eyesight, and my Hashimoto disease. For me fasting is my yearly (several times/year) process of maintaining my youth.
Huge fan here ... it is so much easier to fast now than several years ago. I don't think twice about it much anymore I just do.