Another thing I like is mugi-cha, the barley tea. It has no carbs at all. It's not sweet, and it's pretty refreshing.

I guess it's more popular in the summer, but you can find it in any grocery store or convenience store.

It has the added benefit (if you need that ) of having a slight laxative effect if you drink enough of it. I go through at least a liter of the stuff a day.