As I'm living in the north of Germany I could probably easily go to Denmark and Sweden, haven't tried that yet though. Been to Prague by train once, which was okay, as there was a direct connection by EC.
Actually I'm planning to go to Austria this summer, so I've recently looked it up. Plane tickets are more then twice as expensive compared to train for my route. The train takes 11-12 hours (depending on connection) though, which is absolute max for me. So yeah, wouldn't go further than that. (Still looked up a connection to Croatia, that would've been an absolute pain in the a** by train...)
The Swedish sleeper trains between Berlin/Hamburg and Stockholm are pretty convenient, and I can highly recommend them. Apparently there’s one (run by Snälltåget) between Stockholm and Dresden now as well. When the Fehmarn tunnel opens in a few years, the trip will become a few hours shorter, hopefully opening up new routes.
How long's flying though? Considering I regularly do 1.5hr flights for work, but realistically the whole process takes about 4.5hrs to get from A to B (including the 20 mins drive from the airports). Feels like half my day is gone but I barely get two episodes of Scrubs in on the laptop before it has to shut for landing.
The flight itself is 1.5 h as well, but I would need 2 additional hours by train to actually get to my destination. So idk maybe 6 hrs in total? I have no idea if that's realistic though, have never seen an airport from the inside.