Reusable Water Bottle
Reusable water bottles are worth their weight when you travel. Throw your empty water bottle of choice into your carry-on, then fill it up post-security at the airport (if you prefer to avoid water fountains, just ask at a cafe or restaurant) and you won't have to depend on the flight attendant to keep you hydrated. When you arrive at your destination, your next refill is as close as the tap, as long as the water is potable. Even if the water is questionable, you can opt to purchase one large plastic jug of water instead of dozens of smaller ones and just refill your reusable bottle as necessary.
Not only is a reusable water bottle sturdier (and it leaks less) than a convenience-store bottle, it also keeps your water tasting more like water and less like hot plastic. For travel, consider getting a slightly smaller water bottle that fits easily in a purse or bag. There are plenty of aluminum, glass, stainless steel, and hard-plastic options in the 12-ounce range.