On daytrip from a country to another, I list some ways (my experience) as follows-
1. Amersterdam to Luxembourg city (by train)
2. Strasbourg to Basel (or Germany's Freiburg) (by train)
3. Russia's Blagoveshchensk to China's Heihe (by 15 min boat ride)
4. Vennia to Olomouc (by train)
5. Vennia to Bratislava (by train)
6. Turkey's Bodrum to Greece's Kos (by ferry)
7. Copenhagen airport to Sweden's Malmo (or Lund) (by train)
If 2-day overnight trip or 1-day flight trip are included, more foreign cities can be visited.
By the way, Austria's Innsbruck is also worth visiting.
Thanks, point 5 is especially useful for this trip. I am working on it.