This is a simple one, but an important one nonetheless: reusable water bottles.
This is the water bottle I took with me to pilates today. It always amazes me how many people at the gym have disposable plastic water bottles, whether they brought them or they buy them from the vending machine there. To me, there is no simpler way to reduce plastic consumption than by bringing a reusable bottle with me to the gym, on hikes, on trips (just make sure that there isn't any water in it when you bring it through airport security!).
Do you use reusable water bottles? How else do you reduce your waste?