As long as you are not weak, dizzy, or suffering from low blood pressure rebounding can be very helpful during a fast. You can make rebounding very gentle, even sitting on the mini-tramp and just bouncing can be of value. Rebounding helps improve circulation of both the blood and the lymph. This is important for detoxification on a cellular level. You also might want to look into dry skin brushing to help move lymph. There is a lymph cleansing forum on Curezone that can provide you with a lot of good information and helpful links.
Use your own judgement on how much rebounding you do. Just a few minutes can be helpful. Never over exert yourself when water fasting. It can do more harm than good. Gentle rebounding, slow steady walks and simple stretching can keep up circulation and muscle tone during a fast. Strong exercise can damage the muscles in some people and lead to an increased possibility of fainting, muscle wasting, and electrolyte imbalance (if sweating a lot). Since each individual is different, it is impossible to give universal advice, but always be careful and start slowly with any form of exercise while fasting.