This is my favourite rehydration beverage during / post karate training. It is also a favourite of Mr J! You can have it neat or use ginger and or mint. It is also nice occasionally to use grape juice in place of or to replace some of the water. Alternately frozen grapes can be added. Just put everything in blender and blitz until slushy.
2-3 cups frozen watermelon chunks
water (enough to allow blending) about 2 cups or more is generally ok
Optional add ins and variations
handful frozen grapes or frozen raspberries
1/2 teaspoon ginger
small bunch of fresh / frozen mint leaves
sub in part grape juice for water for sweeter slushy (when not using grapes).