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-Two water bottles. TWO, people. If not three. If you require more water once you're on the plane, it'll be poured from a single-use plastic water bottle. If you bring bottles of your own, you can fill them up after security from the water fountains in the airport and skirt the offered beverages on the plane entirely. Two bottles typically works for me, though on my flight home from Ireland last year I succumbed to asking the flight attendant for a refill after I finished my second one. Though I usually drink from a metal or glass water bottle at home, plastic Nalgene bottles are my travel choice because they're so much lighter when I'm carrying multiples. If you don't own plastic bottles of your own anymore, you could borrow them from a friend or simply swipe empty Gatorade bottles from the recycling bin at work.
Trial and error will likely teach you the number of bottles that works best for you. On the flight to California, my boyfriend only brought one; though I very annoyingly cautioned "You might regret it!" (I am sometimes quite the zero waste know-it-all), he didn't end up needing more. So maybe I'm more of a water guzzler than most.
Another note, beverage-wise: if you're a coffee drinker relying on getting one at the airport, you'll of course want a reusable thermos with you, as well. I like both the Klean Kanteen and the KeepCup.
-Snacks and meals. Airplane food is universally fairly gross, so this is an area with multiple fringe benefits: you can reduce your waste while eating things that are overall more tasty and healthy.
A flight attendant friend of mine recently confirmed that, no, if you refuse the provided airplane meal, it won't be reused on down the line and instead will become waste anyway. I'm curious if any reader has ever called ahead to say "no" to a meal in advance - but I doubt it would make a difference. Knowing this, you could just opt to eat the food instead of letting it go to waste; I don't like the way it tastes, though, and opening all those tiny plastic-wrapped packets now kills my soul, so I prefer to bring my own when I can. Packets of pretzels and soft drinks are, of course, used on down the line if they're refused, since they're not perishable. (Although anyone who has ever eaten dinner on an airplane can attest that those don't exactly seem perishable either).
Regardless, I like to bring my own food, bought in bulk or prepared at home in advance. Things I've eaten on planes recently: homemade sandwiches, bagels bought package-free, apples, citrus fruits, trail mix, pistachios, stir fry over rice in a stainless steel container, a croissant, a big kale salad with grains and beans, chocolate, homemade granola bars. If your flight is quite long, you'll want to prep multiple meals, which can be a pain, especially when you're rushing around doing everything else that's required to get ready for a longer trip. I got a bit lazy before my flight to Ireland last year, and let me tell you: eating peanut butter-and-apple sandwiches for three meals in a row was not an enjoyable experience, even with the addition of chocolate chips to one of them.
If, as I like to contend, zero waste is largely a matter of planning ahead, next time I'll improve matters by jotting down some meal ideas in advance and then prepping them the day before, not the day of. A sample menu for an overnight flight could look like a stir-fry made ahead to eat for lunch at your gate, then a sandwich and roasted veggies for dinner on the plane, followed by a bagel with peanut butter and a piece of fruit for breakfast on the plane the following morning. Planning meals in descending order of the refrigeration needed is key. And supplementing these with copious amounts of snacks, of course, is another way to quell the ennui of a long flight.
A note: if you're leaving the country, customs can make food planning more difficult. If you'll be going through customs upon arrival, make sure you've eaten all your food during the flight, or disposed of it once off the flight. With this in mind, you'll probably want to bring things that don't need composting, since you won't be able to get apple cores and the like through customs. Another tricky scenario to be aware of is that some international airports abroad do U.S. customs before the flight leaves (this happened to me last year in Shannon, Ireland), so you won't be able to take any fruits, veggies, meat, dairy, etc. on the flight at all. If this happens, I recommend using a cloth produce bag to purchase a few pastries at a coffee shop post-customs (said coffee shop probably won't sell any fruits or veggies because it's post-customs) and then eating the airplane food itself. Things happen, c'est la vie.
-A container for compost. I sometimes forego this, instead planning to put my compost in my empty food container once I've finished eating. This of course doesn't work if you don't eat your whole meal or snack at once, so I appreciated having this small reusable bag handy for apple cores, pistachio shells, and the like. A huge bonus was that we were flying to San Fransisco, whose airport has compost bins everywhere. Score.
-A cloth napkin and an eating utensil. You probably already bring these most everywhere, and a flight is no exception. How else are you going to eat your lovingly prepared stir-fry?
-A handkerchief. Indispensable whether in the air or on the ground. But especially on long flights, that dry air can do weird stuff to your sinuses. Don't get caught snatching your neighbor's drink napkin to blow your nose, or using your cloth napkin that you'd prefer to save for other purposes. If you don't already own a handkerchief, you could make your own pre-flight with this tutorial.
Also imperative, though not pictured: warm clothes and maybe a scarf or blanket and a pillow of your own. Anything the flight attendant might hand to you, you could try to bring for yourself to avoid the inevitable plastic wrapping that accompanies it.
Though I typically bring all of the above, I could share plenty of stories when my best-laid plans were rerouted along with the airplane and I ended up buying food in a compostable or recyclable container, accepting a beverage from the cart, etc. If this happens to you, you'll learn something, you'll try again next time. Over time the lessons stack up and I get better and better at zero waste traveling. If you have lessons of your own to share, I want to hear: what else do you bring for flights like these?
Previously in Travel: A trip to Rainbow Grocery, and how to compost while traveling.