Koper

If you've never seen a major show cave, yes — Postojna is one of the largest accessible cave systems in Europe and the tour runs on a small electric train through the chambers. It's about 60 km / 38 miles from Koper, roughly 45 minutes by car. Adult tickets are €30.90 in peak season (June–September) and €27.90 off-peak, the tour lasts about 90 minutes, and English-language tours run hourly from 9am to 5pm in summer (with a 6pm slot added in July and August). Book online a day ahead in summer or you'll watch your slot fill while you're waiting in line.

Piran is the prettier town. Koper has the dock and the Venetian bones; Piran has the actual Venetian skyline — Tartini Square, the campanile that's a smaller St. Mark's, narrow lanes that end at the sea on three sides. It's 11 km / 18 km by road from Koper, about 24 minutes by car or 43 minutes on the Arriva bus from Koper's Tržnica station (every 30 minutes, around €4 one-way). Reasonable cruise-day move: morning in Koper old town, bus to Piran for lunch, back by 4pm.

Possible but tight. Ljubljana is 82 km / 51 miles from Koper — about 1h 15m by direct GoOpti bus (around €10–15 each way) or 2h+ by train. Realistic only if your ship is in by 8am and out no earlier than 8pm; most calls don't give you that runway. If you have to choose, Postojna Cave is closer and more cruise-friendly. Save Ljubljana for a land trip.

Plaža Žusterna is the closest swim — a concrete-platform Adriatic beach about 1.5 km southwest of the old town along the seaside promenade, walkable in 20 minutes or a quick taxi. It's not a sand beach (very few are on this stretch of the Slovenian coast); it's the Italian-Adriatic style of swimming off paved decks with ladders into the sea, plus an Olympic seawater pool that's open year-round. Useable June through September; August water hits about 26°C.

Slovenia uses the euro and has since 2007. Cards work everywhere that matters — the cathedral entry, restaurants on Tito Square, the bus station ticket window, Postojna Cave online and on-site. Carry €20–40 in small notes for the Arriva bus to Piran, market stalls, and the occasional small-town café. ATMs are easy to find in central Koper; decline the dynamic-currency-conversion offer and pay in euros.

Yes — Koper is small and licensed cabs queue at the terminal exit. Slovenian taxi base fare is around €1.70 with €1.10 per kilometer; a ride to Piran runs roughly €25–35 depending on traffic. Confirm the meter is on before you start, or agree a flat rate up front for longer runs (Postojna, Ljubljana). Uber does not operate in Slovenia, so don't plan around it.