I took the bus from Munich to Ljubljana last July. It was comfortable and reliable. Probably also a scenic ride, but it was a very long vacation with many buses... i think I bought the ticket the night before. That's generally a good idea with border-crossing buses.
Bus service from Ljubljana to Bled is frequent, and Lake Bohinj is reachable by bus from Lake Bled. The tourist office in Ljubljana is excellent and can help with transportation options for other destinations. Buses definitely go to Piran, in the Slovenian part of Istria, and less frequently to Rovinj in Croatian Istria. Rovinj takes too long to be a day trip, however.
The Skocjan Caves involve a bus ride followed by a walk. The Postojna Caves are probably more accessible.
I found the taxis in Slovenia quite reasonable last year. There's often one hanging out around the Bled bus station, looking for fares to nearby beauty spots or back to Ljubljana. In that situation the cabbie will be trying to fill his car or van, and the cost can be very low. For trips within Ljubljana the T.O. told me it is cheaper to call for a cab rather than grab one off the street.
Edited to add: English is spoken widely and we'll by a large percentage of the young people, including at the Ljubljana bus station as of last summer.
I've also tweaked,the phrasing in the first paragraph so it no longer suggests that I took a lot of buses to get from Munich to Ljubljana.