What are good ways to flavor water?
Lemon juice
This one is fairly obvious and self-explanatory. Although the idea is mainly associated with highly sugarey lemonade, just a few drops of juice in a bottle of water and no extra sugar gives it a nice touch.
Vinegar
A matter of taste. Many will find this just gross, but it has of course a similar tartiness as lemon and less sugar.
Cucumber
This surprised me a lot when I first tasted it: you'd think cucumbers hardly taste of anything at all. But put a few fresh slices in a jug of water for half an hour, and they'll impart a subtle, but astonishingly interesting and really quite refreshing flavour.