This is so simple! Depending on when you plan to drink it, chose your favorite bag green or herbal tea ( I like green in the morning and herbal for after 3-4ish in the afternoon).
Just boil one liter of water, brew with your teabag of choice and let cool off. Add fresh ginger, lemon or fresh mint according to taste. Let cool in the fridge overnight and enjoy the following day. In the summer addig ice cubes would be a nice touch but now in wintertime its really not necessary. Keep green and hydrated, enjoy!
Herbal teas are a great way to keep hydrated.