I had a somewhat limited view of Arenal, as I visited it as a day tour from Playa Hermosa. The trip up the mountain from the main road is slow with some potholes, but very scenic and well worth the drive. However, our day, it rained and the volcano was not very visible, but we did enjoy the hot springs near Tabacon Hot Springs - sitting in the hot spring with rain falling is quite an enjoyable experience, as you did not need to get out to cool off under a shower. Visitors that stay overnight at Arenal are more likely to see the volcano erupt at night when it is clear. There are hotels there with balconies that face the volcano and have a wakeup call service for guests wishing to see volcanic activity. There are various hotels and B&Bs for all budgets and after speaking with other travelers who made the trip, it is highly recommended that one stay overnight.
The volcano is frequently in the clouds by afternoon and the best viewing is night or early morning. By the time most day tours arrive from San Jose, the mountain is clouded over.