We didn't need a car our entire time in Arenal. Getting to Arenal from San Jose was simple with a van ride that picked us up right at the hotel, the Best Western. Every daily excursion we took was booked through whatever hotel we were staying at and each included transportation. It was much more relaxing to let someone else do all the driving.
If you plan to stay in the area you may wish to drive, however it you are planning a daytrip I would suggest doing one of the tours. For about 80 USD (lunch & dinner included) you get a full day of fun. Most leave in the morning, and make a few interesting stops along the way, getting you to Tabacon Hot Springs about mid afternoon. You will have plenty of time to soak in the water, walk the paths, and they even include a full dinner in the restaurant. By the time you finish eating, you are lucky, you will get a chance to see a night eruption. After about 10 hours of fun, they return you to the hotel.
The picture was taken from the tour van, on the way up.
The other side of the lake we were met by a 4WD vehicle which was to transport ud the last part of the journey to Monteverde. Unfortunately, there wasn't really enough room for four large people plus luggage, so the trip was rather cramped and uncomfortable.
The scenery though, was really spectacular, and the tracks were just that - dirt tracks! We crossed several bridges which looked as if they would not hold the weight of the vehicle, but they did!
From Arenal we continued our journey to Monteverde, which first of all included a boat trio across Lake Arenal. It was raining heavily the entire journey, so unfortunately we weren't able to enjoy the magnificent scenery to its full.
The drive from San Jose airport took me about 3 1/2 hours. Since the roads don't really have names or route numbers, you have to know what town to drive towards.
The first target is the town of San Ramon. This will take you about an hour from the airport. Take the main street in San Ramon to the end, make a left, and right at the light. Your next target is Los Angeles, and then La Fortuna. You will see signs along the way for Arenal Volcano. This drive is not bad. But you still have to pay attention and try and avoid potholes.
From La Fortuna it will take you 15 minutes to Arenal, depending on where you are staying.