How to get to your destination is also important. There's a lot of things said about flying, and you may hear things like this:
Flying gets a bad rap, but airplanes become more efficient the longer the flight because high-altitude flying is more efficient. In contrast, hopping onto a short-haul flight is as bad as crossing the country alone in a gas-guzzling truck. -- fix.com
This does not mean "flying is not that bad" it means "if you have to make a certain trip, sometimes a flight is more efficient than driving a heavy truck alone", but if you don't need to make the trip or can get on public transportation, that is always going to be better.
Getting around most of Europe by train is the way to go. Even if trains are a little slower, it's ok because night trains have made a comeback! You can get wherever you're going while you sleep, and that's one less night in a hotel. Multi-day trips include food in the price, so you can avoid the temptation to splurge on a fancy holiday meal. The prices are higher than flying due to subsidies for airline companies and lack of subsidies for train companies, but that is starting to change too. Germany reduces train tickets by 10% in 2020 and Austria is raising taxes on flights.
Cruises generally use diesel propulsion which contributes similar greenhouse gasses to flying, so avoid cruises at all costs. Whilst some ferries are switching to electric, most have not done that yet. Hopping on a cargo/freight ship or ferry which is already transporting a bunch of trucks means you can make hops across various at 10% the emissions of flying. Releasing gases at high altitude has a drastically increased warming effect, so let's bring back the transatlantic ship crossing: it will make us consider if we really need to go half way around the world for a quick vacation, or meetings which probably could have been virtual calls.