My husband tried making an isotonic drink using a recipe he found on a site associated with the UK institute of sport (or whatever it is called). It was foul. If I recall correctly, it was a combination of salt and sugar and water.
We have had a lot of success with diluting fruit juice, however, here in Australia we have a brand of apple juice which naturally contains 13% sugar and we dilute this 50/50 with water to give us a 6.5% sugar concentration. We haven't actually tried adding salt to the juice but I guess if you have a fruit juice which is 6-8% sugar or can be diluted to that level, you could try adding salt and see how it tastes.