Don’t litter.
You can at least control the trash and waste you create yourself. In Ontario we used to have "Keep Ontario Beautiful" on vehicle license plates. I miss seeing it there. But, the slogan was all about appreciating what we have and keeping it clean and looked after.
Reuse plastic bags, they are great for repacking when you have dirty clothes and your walking shoes.
Walk.
You see more when you walk anyway. But, you also make a much smaller carbon footprint. For an adventure explore other ways of getting around that don’t rely on fossil fuels. Other places have different situations and weather conditions, you could have a great day exploring on horseback, renting a bike, or even taking the local bus (at least you’re not consuming extra fuel resources that way).
Eat In.
Shop at the local stores and pick up whatever fresh food you can find. Fresh food will have less packaging and create less garbage.
If that isn’t practical you can still buy food from the grocery store and have a picnic outside or bring it back to your hotel and eat indoors. Give support to the local economy, save yourself some money and don’t eat food that has been imported from "back home" when you aren’t at home.
Power Off and Save Water.
Energy and water are world wide problems. Be energy and water conscious even when you aren’t at home and won’t see the bill at the end of the month.
Take Tour.
Being in a new place can be intimidating. Make sure you get out, take a tour recommended by your hotel, site outside somewhere and people watch, go for a walk. Make sure you see more than your hotel before you head home.
Pick up a map and find local attractions, historical places, art galleries. Some museums and galleries will have free admission days during the week.
Be Careful What You Buy.
Chances are any natural exotic product is going to be a problem when you cross the border going home. Exotic animals and plants are a big NO, even if they are no longer alive. Understand the rules before you start spending your money.