Getting around most of Europe by train is the way to go. Trains have a reputation for being slower than flying, but when you factor in mucking around at the airport either side, and getting to the airport, many trains trips under 500km are actually quicker than their equivalent flights. you’ll be happy to know overnight trains are making a comeback. This might sound like some old-timey Agatha Christie nonsense, but sleeping your way to your destination is an awesome way to remove the stress of flying, and knocks the price of one hotel night off the trip too. Instead of fighting through a million lines, taking your shoes off, unpacking all your electronics, and being violated by those naked scanners, you can finish work, casually womble to the train station, and go to bed.
"Budget airlines" have made flying unnaturally cheap due to the hefty subsidies they receive, but that is starting to change too. Germany reduces train tickets by 10% in 2020 and Austria is raising taxes on flights. Budget train company Flixtrain are offering tickets all over Germany for €9.99, which can make for some interesting options when you pair it up with Nightjet and other European train providers.
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. 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.