the main electrolytes are calcium, magnesium, potassium, and sodium. most people get plenty of sodium, but most lack potassium which is why i recommend it. water soluble cal and mag are hard to find thats why i recommend black strap molasses which is high in all the minerals.
sugar is another thing you lose while sweating, thus adding some potassium and a mineral rich sweetener to your water (like molasses or sorghum syrup or maple syrup) would help out a lot.