The journey from Manuel Antonio to La Fortuna is quite long, and public transportation requires going back to San Jose and changing buses. So that's why I decided to book a shuttle service that goes directly, in an air conditioned van. While the service sounded very good on paper, the reality was different: the van didn't show up at all. When the hostel owner called the company, they told him that our reservation had been canceled - without notifying us at all. The afternoon service was already full (or so they told us), and they refused to do anything more for us. I argued with them for a little bit and threatened to write bad reviews online - to which they said, 'yes, ok' - but all my arguing had no measurable effect. The only thing I was amazed at was that my Spanish was good enough to understand them and argue back :-)
So between the options of public buses and staying another day in a place where we had seen pretty much everything, we chose public buses - although I wasn't quite certain we would be able to make the trip in one day. The first part, from Manuel Antonio to San Jose, was quite relaxed: the bus wasn't full, and we had plenty of space. In San Jose, we took a taxi to get to another bus terminal for buses headed north. The taxi driver had bad news for us: all the direct buses to La Fortuna had already left in the morning. We would have to go through San Carlos and change buses once more. As luck would have it, the next bus to San Carlos left within twenty minutes of our arrival. The trip was a little bumpy - bus drivers speeding through winding mountain roads usually have this effect ;-)
San Carlos only has one bus terminal, so we didn't have to change. And we got lucky again: a bus to La Fortuna was due to leave in half an hour. Enough time for a toilet break and to buy some water. Another ninety minutes later, we finally arrived in La Fortuna, where we took a cab to our hostel.
So, after nine hours, three buses and two cabs, we had successfully completed the trip. In comparison, the shuttle would have taken six hours and left two hours earlier - but we would have paid more than double for it.