The recipe: A heaping 1/8 cup of maltodextrin (or 50-100 Cal of any maltodextrin product like Gu, etc) diluted into 16-32 oz of water. If using maltodextrin without any protein, sucrose or fructose already in it, add an optional 1/8 cup of baby formula and 1 teaspoon of sugar (or sweet chai powder mix).
The theory: The most important molecule besides water during exercise is glucose. Dextrose is two molecules of glucose attached to each other, and maltodextrin (“malto”) is a chain of a dozen or so glucose sugars. For the same number of calories, malto is a little easier on the stomach so I generally recommend malto diluted in water as the most basic first principles sports drink. Malto is a fluffy white powder ~100 Cal / quarter cup that can be purchased in any home-brew supply shop, or in the pure malto products CarboGain, CarboHit, or CarboPro. It comes with some other (unnecessary) ingredients added in Sustained Energy. Malto is also sold as syrup (with a little water and sweeteners added) in the form of Gu, Powergel, Carboom, e-Gel, and Hammergel, any of which can be further diluted into water. I recommend 50-100 Cal of malto in 16-32 oz of water for any exercise of moderate to high intensity lasting more than 30 minutes. Use twice these amounts (of malto and water) if training is longer than 1 or 2 hours, depending on the intensity.
Sometimes I just go with the basic recipe of malto in water, but often I also include baby formula for its protein and lactose (glucose and galactose) content, as well as table sugar for its sucrose (glucose and fructose). Protein reduces the need for the body to scavenge amino acids from muscle during training (this is theory; it has not been shown to have a real impact in athletes), and galactose and fructose supply fuel to the liver to maintain long-term energy reserves (for long workouts or to promote recovery even before short workouts are completed). The healthy fats in baby formula will, in theory, slow digestion slightly and introduce recovery fats into the body, so that is an added bonus, although not of great significance. I use baby formula since it has all of the ideal theoretical ingredients, not because these ingredients actually have measurable impact on performance. That is part of my punishment for getting a PhD in chemistry.