IF STAYING IN A HOTEL
Choose a place that holds itself to high environmental standards, ideally certified by a third party like Rainforest Alliance or Global Sustainable Tourism Council. And research a hotel with a local owner instead of a big corporation.
And treat it like you would your home when it comes to swapping towels, turn off your TV and lights when you leave.
ALWAYS TRAVEL WITH REUSABLES
just like you do on your day to day (we hope!) use a reusable water bottle, a coffee mug, a cloth shopping bag and reusable straws and utensils.
Normally when you travel you eat out and shop more so plan ahead when going to markets, restaurants and buying souvenirs (and make sure you only buy souvenirs that you would actually use!)
RESEARCH YOUR DESTINATION
find out if the place where you are going has a good public transit system, or if it's pedestrian and bicycle friendly and consider these options before choosing to rent a car.
Be sure to shop and eat local as to contribute to the place where you are.