>High speed trains are a good alternative (for domestic travel)

This is the only real answer. There's a hard limitation to how emissions friendly aviation can ever be. The energy density requirements given current technology can only be met with hydrocarbon fuels. High speed trains are infinitely more efficient, and actually would have an outsized effect on emissions given that the type of short haul flights it would replace are the least efficient per mile travelled, since takeoff/climb uses far more fuel than cruise. We'll never be able to replace carbon intensive long haul international flights, but focusing on what can be replaced could go a long way.

I fell in love with train travel in eastern europe. Get on a train at night, curl up in your bunk to get gently rocked to sleep by the movement of the car, wake up in the morning in another city refreshed and ready to go. It's the closest thing to time travel that I've ever experienced.

Not having to deal with the security theater is also a big plus.

When you take in account all the time it takes getting from your place to the airport and back to the hotel there's usually not much of a difference in time in taking the train anyway.

Or international if you're in a small country ;). My wife and I take several multiday trips to places in Europe by train instead of flying to a faraway place once (or twice). Better for the environment and it's not like there's not enough to see and do nearby (ish) for several lifetimes.