Carry Your Own Coffee Mug/Refillable Water Bottle at All Times
I have one refillable water bottle that lives in my backpack (which is basically on me at all times), so I never need to purchase a bottle of water. I refill it at airports, casual restaurants, or anytime I spot a water fountain. The same goes for coffee mugs, too. Bonus: many coffee shops will actually give you a discount when you supply your own mug. It's a win for you and Mother Nature.