DIY Electrolyte Drink
Sometimes rehydration is more important than fueling, especially in the summer. If you want a low calorie electrolyte you can buy Nuun tablets or similar options, these are a little pricy and don't contain enough sodium. A far cheaper option is to use the sugar-free single serving packets that are designed to be added to a 16 ounce bottle of water. I rather like the Crystal light green tea mixes, so any flavor will do. Simply mix a packet with 32 ounces of water and add a quarter to a half a teaspoon of ordinary table salt. I find that diluting the packet to twice the recommended volume works well when you're adding extra salt. You don't need any other electrolytes, though you can add a pinch of 'no salt' if you'd like a little extra potassium.
1 Relationship to Go Juice
You can add more salt to this drink than you can to Fellrnr's Go Juice as the extra ingredients in the Go Juice compete for absorption (more precisely they increase the osmotic pressure). Also, Fellrnr's Go Juice contains a lot of calories, sometimes more than you want if you're just trying to rehydrate.
2 See Also
If you want to know more about hydration and why the extra salt is required, read Practical Hydration and for the underlying science read Hydration 101.