I add chopped mint leaves (fresh, not dried!), slices of fresh ginger and slices of fresh lemon to my water. I make up a bunch of bottles and have them in the fridge ready to go. The mix makes my water taste lovely without adding sugar, articicial sweeteners or acids that would upset my stomach.
If you don't like stuff floating in your water, you can mix it up in a big pitcher and strain it as you pour or place the items into an empty tea bag (available at most health food stores).