I always have one bottle of plain water (luckily we have great water here in Northern California). If it's a multi-bottle ride then some kind of electrolyte replenishment drink goes in the second bottle. In my experience, not using electrolytes can result in horrendous cramps after 50 or so miles on a hot day.

If it's a killer ride like the Deathride or a double century then the plain water bottle gets a carbohydrate mix (e.g. Maertens) put in it roughly every other bottle fill so as to avoid bonking. I really prefer not to do that because I really don't like the taste and texture of that stuff.

I find it surprising that so many people in the thread above talk about putting a non-nutritious chemical mixture (Crystal Lite) into their water purely for flavor unless the water is really foul tasting. Water is what keeps us alive.