We did not take the GC post trip to Lake Bled. Instead, we did our own post-trip to Slovenia using the Rick Steves guide book for Croatia and Slovenia as a reference. We took the train from Zagreb to Ljubljana, picked up a rental car, and spent four full days exploring Slovenia. While exploring Ljubljana and Lake Bled, we kept running into our friends who were taking the post-trip extension. Grand Circle offers the optional visit to Postojna Caves. We did not visit these caves; so we can not comment on them. We did go to Scocjan Caves, which is in the general area. Scocjan Caves were very similar to Mammoth Cave NP here in the USA. Our reason for picking Scocjan was because we could combine those caves with a visit to the nearby Lipica Stud Farm. The Lipica Stud Farm is where the Lipizanner horses are raised and trained. The two places made a good day trip.
Our tour leaders for the main trip were Sinisa and Jana -- both were excellent. Here's a tip: If you need to do laundry sometime during the trip, there is an excellent place in Split called the Modrulj Laundrette. They provide full service for just a little more than self-service. The place is about a ten-minute walk from the ship. We dropped off a load of dirty clothes late in the afternoon and picked up our clean clothes the next morning after the walking tour of Split.