Tour in Trieste before heading to Venice

[RoLyWe]

A few friends and I will be sailing into Trieste in July (NCL Escape from Barcelona) and would like to venture into Slovenia to visit Postojna Cave & Predjama Castle for a few hours before going up to Venice. Our concern is our luggage. We will have one check and one carry-on bag each. From what I have read the only storage is at the train station and I don't know how much time we would have after getting off the ship prior to the tour pick up. Does it make more sense to just rent a car for the day? The castle is only an hour from port. After the tour we would plan to take the train up to Venice. Has anyone taken this tour, and what are your thoughts about luggage storing for 5-6 hours?  Thank you in advance.