Bottled water
Unless you happen to live in a city where the water is unfit to drink, there is no reason to buy bottled water. The water from your tap will hydrate you just fine. Invest in a couple of reusable water bottles, and fill them for cheap at home.
Water tastes iffy? Buy a water filter or a filtering pitcher, and keep it in the fridge for when you want a cold drink or need to refill those reusable bottles.
If you must have bottled water from a store, buy it at your warehouse club.
