Air travel is without a doubt the most environmentally damaging part of travel. Passenger planes on average produce 244 pounds of carbon dioxide every mile. So, easiest way to make a difference? Limit your air travel as much as makes sense. Travel overland or by boat. Take a bus, or a tuk-tuk, or a camel, or a dug-out canoe, or your own 2 feet. Not only will you contribute less CO2, but you will definitely see more of the area you’re traveling through, and might even get a great story out of it!
I get that cutting out airplanes altogether is pretty improbable for a lot of trips because of the time they save over other methods of transportation and the comfort they afford, but even just switching one leg of your trip to a different mode of travel will cut down on the amount of CO2 your trip produces.