The regular route from La Fortuna to Monteverde is about 8 hours by bus. This takes you around lake Arenal (which is quite large) through the mountains. To cut this short, there are boat rides across lake Arenal.
We started from the hotel at 8 am and took crossed lake Arenal on boat. This was a very scenic ride. Views of rolling hills, huts, cattle (very sparse), clear sky, blue water all around made the ride very enjoyable. We met an Indian guy who was on a 6 month Central America trip (who we bumped into again and again during our trip).
After the boat ride, all our co-passengers took a van to Monteverde. However, we took a jeep into the mountains. This was the Costa Rica back country. The muddy roads (they were more like trails) were very rocky, steep, with extreme curves. The all wheel drive bulldozed through those roads. It was a very bumpy ride, and many times we were on the edge of taking a tumble. I had some conversations in broken Spanish with our driver Fermang (It was more of a sign language than actually speaking). Yak pulled out the Spanish dictionary for help.
The jeep took us as far as it could, and beyond a point it was not possible to ride a jeep. Here we got on to horses (which did not look very strong) and set out on one of the most adventurous rides. The trails were along the mountain slopes most of the times and were narrow. The recent rains had made them muddy and the horses sank knee deep into mud most times. Other places it was quite rocky. We cross forests, streams, and farms. The horses would compete to get in front, pushing each other out of the way. All this would happen on narrow steep trails (60 deg grade) / rocky / slushy, with deep valleys on both sides. A couple of feet to either side are you'll have a free fall. Our adrenalin rush matched that of the horses'. One trail after another, and another, for 3 hours. This was the most adventurous thing I've done after the underwater cave swim in Jamaica. Our guide Rafael did not understand a word of my Spanish, and I quit trying after a point. Finally we arrived at a hotel on the hills by noon.
We had a brief stop there, a sandwich for lunch, and then headed to Monteverde is a jeep. On the jeep, we met two brothers from the US (photography enthusiasts) who have been trying to get a picture of Arenal mountain for 5 days (and a week during their earlier trip) but have not been able to do so. We learned that a good lava sighting is rare, and we were among the lucky few who got a view. The jeep dropped us off at the Tree Top hotel in Monteverde.