Koper is the largest town on Slovenia's short Adriatic coastline, and one of the country's most underrated cities. Its compact medieval center sits on what was once an island, surrounded by the sea before land reclamation connected it to the mainland. The old town's Venetian-influenced architecture — Praetorian Palace, Cathedral of the Assumption, narrow stone streets — makes it a pleasure to walk through, and it rarely draws the crowds that hit nearby Piran.
The centrepiece is Titov Trg, the main square, surrounded by some of the finest Venetian Gothic civic architecture outside Venice itself. The Loggia Palace, modelled on the Ducal Palace in Venice, closes one end of the square; the Praetorian Palace commands the other. Climb the Cathedral Bell Tower for views over the terracotta rooftops to the Adriatic beyond. The Koper Regional Museum in a 17th-century palace nearby covers the complex history of a region that has been Venetian, Austrian, Italian, and finally Slovenian.
The city is also Slovenia's only commercial port, so there's a working edge to it alongside the historic centre. Trieste is twenty minutes north, Piran is forty minutes south. Koper makes a good base — and a more authentic one, with a genuine local community, than the almost entirely tourism-oriented smaller towns along the coast.