In reply to Stone Muppet:
If you can stop and have a snack, as opposed to continuous running, you're not limited. It might be better to take a bag of peanuts, so you get salt, carbs and protein.
But you asked for something to add to a drink. I use SIS Hydration tablets, which taste nice and come in several flavors. You can add the recommended amount to your water bottle (2 per liter), which is probably designed to rehydrate someone doing short runs.
What I do in ultras is add a lesser amount (~1/ per l), as too much salt will act as a diuretic. It's probably better to get some electrolytes over a long period, rather than trying to get the maximum amount all the time during that period. Also, you'll be filling up from streams and it will be hard to measure the exact concentration.