These small containers that can fit in a cartridge box have saved my behind on many an occasion. Electrolyte tablets! These little wonders are like Alka-Seltzers. You pour yourself some canteen water into your tin cup, plop one of the quarter-size tablets in, and it fizzes up and within a minute you have an electrolyte drink similar to Gatorade.
They come in several different flavors. The fizz dies down if you let the tablet dissolve completely and it's just a regular flat energy drink. I highly suggest a soldier carry one of these. The carbonation is very light and it disperses quickly, so I don't think it will hurt the seams on your tin cup or the inside of your canteen. (I've used it at every reenactment and I haven't been poisoned yet!) it's surely much easier to carry around and hide in your kit than bottles of Gatorade, and it has no need to be kept cold.
My only caution is wait until they fully dissolve or you could get a bad taste. Don't eat the tablet without water, it will be nasty and make your tongue feel super weird, not to mention taste awful. Just drink the flavored water and it'll give you what you need.
The plastic bottles are small, really they're slightly bigger than the size of a 14 gauge shotgun shell.
You can pick these up at a retail place like Dick's Sporting Goods. $6.50 gives you 10 tablets, but It's worth it.