Quick Answer
Postojna Cave is an easy Ljubljana day trip. Public transport can work well for the cave alone, while a car or guided tour is often smoother if you also want Predjama Castle.
Before you travel, check current cave tour times and the connection between the cave and castle. The two sights pair naturally, but the logistics are not the same as visiting one city-center attraction.
Transport Options
Simple public transport
Bus
Usually the easiest car-free choice if times line up. Check whether your stop is convenient for the cave entrance.
Rail travelers
Train
Works when schedules fit, but check the final walk or local connection to the cave area.
Cave plus castle
Car
Best for combining Postojna Cave with Predjama Castle and avoiding awkward transfer timing.
No logistics
Guided tour
Useful if you want transport, entrance coordination, and the castle add-on handled in one package.
Cave only
Use public transport if you want a simpler half-day or relaxed single-sight outing.
Cave plus castle
Use a car or tour if you want the most efficient combo with fewer transfer worries.
Family note
Bring layers, snacks, and buffer time. Underground tours can feel long if kids are tired or cold.