Put the sugar in the pyrex jug. Boil the kettle. When the kettle boils, cover the sugar with boiling water, stir to dissolve. You don't need to make syrup or anything, but you want the sugar dissolved.
Meanwhile, put the tray of ice into the 2 liter jug. Squeeze the lemons, limes and orange in, getting out all the juice and pulp you can and avoiding adding the pips. Pour the dissolved sugar and water in. Top up with cold water. Shake or stir. Drink, with ice. It'll be cold enough. I used to refrigerate it for a while first, but then I had to make some in a hurry and it was just fine.
This is very refreshing and about as isotonic as you can get. I sometimes add mint or basil to the sugar in the boiling water when I have that growing outside. If it's too sweet, use less sugar next time. I figure this has about a teaspoon of sugar per glass.
The other thing you can do, right now while limes are nine for a dollar, is just squeeze half a lime into your glass of water and ice. Kids won't drink this, but it's good.