Here's the same volcano as we saw it from our room in the morning at the Arenal Observatory Lodge. All day and night you hear booming explosive sounds of hot boulders crashing down the sides of the mountain. Supposedly it's totally safe for us this far away, though a tourist was killed was a few years ago, apparently from hiking too close when the volcano was having a bad day.
That lodge is a great place to stay, by the way. Our family (3 kids) is too big for a single room, but we were able to get two rooms side-by-side in what they call the Smithsonian section of the hotel. The view, as you can see, is spectacular, but that's not all. There's a wonderful "infinity pool" just outside the room, and plenty of nature hiking. The rates are on the order of what you'd pay at a Holiday Inn in a big US city ($100-$150), and include good US-style breakfast food.
The only downside of the lodge is its remote location. It's an hour-long, very bumpy ride up a desolate but well-traveled road. No dangerous cliffs or curves, but you will be very glad when you reach the top.