First of all, it all starts with your packing already. There is a number of choices you can make here that will help you protect the environment and save some money even.
Surely everyone of us has a few plastic bags lying around at home. Simply use one to pack your shoes inside and pack one empty one (they don`t take up much space anyway). Plastic bags are great to pack your dirty laundry, your shoes on the way pack, any kind to liquids to prevent spilling and any other wet things like a bathing suit, umbrella or towel. And when you bring then you can reuse them easily and don`t have to take one from the hotel or a super market at your destination.
Additionally, you can bring your reusable water bottle and coffee cup. In almost every country there are drinking water stations installed at the airport where you can easily (and for free!) refill your water bottle after the security check. And when buying a coffee at your destination, especially when you are on city trips, it is fantastic to have your own coffee cup with you and ask the barista to fill your beverage in there.
And one more thing not to forget: your foldaway shopping bag. I always have mine with me and it`s great for putting your jacket in there once it get warm during the day, to hold your shoes during a walk on the beach or simply your souvenirs when you go shopping.
Also, try to think clearly about what you need and what you can leave at home. This is not only for the cost saving as by bringing less weight every kind of vehicle you are using on your way needs less fuel and therefore accounts for a little less pollution. I know it might only be a small amount, but every bit helps.
When you are at the airport, try not to buy to much prepacked food. It`s mostly not very healthy and uses plenty of plastic. Instead have your home made sandwich in a box, that often comes in handy when wanting to store some leftover food on your journey, and your reusable water bottle. Not having to buy things at the aairport is also going to save you a whole lot of money! And even if you have some waiting time, try not to buy in the duty free area. Things are always packed into see-through plastic bags here, which is a complete waste.
When you have arrived at your destination, don`t opt for buying a ton of small water bottles. Instead go for a pack of a few larger bottles that you can fill into your reusable water bottle which has the perfect size to take during the day. Often you can even use drinking water from the tab and ask in restaurants, bakeries or coffee shops for a free refill of tap water.
A rule that is important everywhere you go: Don`t just drop your waste, but put it in the designated bins. In case there shouldn`t be any around right now, simply take a plastic bag out and collect your things in there for now or make one of the outside bags of your backpack to a designated "bin collection' place until you can put it somewhere else.
Once you are on your way back, you can also do something good for nature. In case you brought travel sized toiletries, don`t simply bin the empty (or half empty) packages. Use them until the very end and then you can still keep the things. I am sure they will find a space somewhere in your luggage as they are absolutely tiny and light weight. They are fantastic for refilling, so that you don`t need to buy any new small plastic bottles and boxes.
I am not saying you are absolutely not allowed to do these things, but I`d like to raise your awareness on how easy it is to actually do something good for the nature. I admit that I myself have sometimes had to buy something at the airport when I forgot or broke something, but for the bigger part I manage to think of the things up on the list