First, flying is the most environmentally friendly way to travel today, as long as you aren't in your own private jet. Lower CO2 emissions/mile than driving thanks to Boeing and Airbus developing new technology over the last 30 years. But, if you want to be environmentally friendly, stay home. That's the only way to do it.
Second, I looked at taking a cruise to Europe and back recently. Price was about the same as round trip plane tickets, plus we would get fed for six days and have a place to stay. It didn't work out because 12 days transit was too long, but if you have the time, a cruise could be great. Plus, no jet lag, which is one of the things I detest most in life.