I am a UK citizen travelling on Austrian Airlines from London to Bangkok, with a connection in Vienna. I have a 7-hour layover.
I am thinking of using the time to visit Vienna itself. I believe that the train to the centre is quick, so I should get at least a couple of hours in the city.
Are there any administrative considerations? For example, if I were transiting through a US city, I would need an ESTA to do this sort of thing. As far as I can see, all I need is to ensure my passport is stamped as I leave the airport, so that when I return for my Bangkok flight, it is clear that I have complied with the Schengen 90-day rule.