I like Nuun, mixed with water. The citrus version adds a great light flavor to the water.
Nuun is very versatile, since there is no sugar in it. For a short ride (or any other physical activity in hot weather), it just gives you basic electrolytes and makes the water taste good. For longer rides, you can use any gel or other energy source of your choice.
No, I'm not a Nuun representative... I just read their FAQ a few times, and like the product.
A better value though, is Endurolyte capsules from Hammer. A bit cheaper, per serving.