Choosing between rail and air
A useful rule: if the train door-to-door time is within an hour or two of the flight once you add airport travel, security and transfers, take the train. You will work, read or sleep the whole way, and you arrive at a station a short walk from your hotel. Milan to Bologna, Vienna to Prague and Madrid to Seville all pass that test comfortably.
Lisbon is the exception. Spain and Portugal have no fast cross-border rail link at the time of writing, and the overnight Lusitânia service no longer runs, so flying is the sensible choice. Copenhagen sits in between: from Hamburg, the train takes around five hours via the Jutland route; from further afield, fly.
Timing the Sunday return
For flights within the Schengen Area, aim to be at the airport about 90 minutes before departure on a weekend. Copenhagen and Bologna have short transfers and allow later exits from the city; Lisbon's metro takes longer and Seville's bus can be slow in Sunday traffic. For trains, ten minutes before departure is usually enough, and in Italy you must board at the coach printed on your ticket.
Lower-carbon weekends
A return flight is often the largest part of a weekend's footprint. Prague and Bologna are the easiest cities in this dossier to reach overland from Central and Western Europe; plan one of those for the next weekend you want to keep light.
Pick your month