Liquid IV. $29/30 pack at Costco. Often found on sale for $20.
It is supposed to be one packet to 16oz of water, but I mix it with 24-32oz of water.
Generally I just drink water, and depend on food to get electrolytes, but for exercise, long stretches of yard work, shooting matches/classes in the heat, etc. I use that, and it helps a lot.