Arenal Volcano looms over the town of La Fortuna. It is an active volcano, and the only one in Costa Rica that is producing visible lava currently. You can only see it at night, glowing red on the mountainside.You aren't allowed to get very close to the lava in the volcano national park for safety reasons, but you can see it from a distance.
You best bet for lava viewing is to stay at one of the hotels on the side of the volcano where the lava is currently flowing. This is currently the north side, and hotels with good lava views that were recommended to us by others staying there include the Observatory Lodge and Linda Vista Del Norte. There are others also.
However, we weren't staying at such a hotel. Our hotel had views of the volcano, but not the lava area. So we took the advice of others and drove around to the dam of Lake Arenal and parked to watch the lava. We were there at a fairly rainy time of year, so the top is often shrowded in clouds. We made the 15 minute drive 3 nights in a row to try to see the lava, and on our 3rd night we got to see it flowing and shooting into the air. (The first night, we didn't really have much hope since it was very rainy.) The clouds would part to give you a view for a few minutes, then close back in, then blow off again a little later. There were several other cars at the dam each night hoping for a glimpse also.
Driving at night wasn't too bad. The road to the dam is pretty good. But don't go too fast! We came around a corner at Tabacon Springs resort once to see a full-size coach bus across the whole road, trying to turn around.
If you don't have a car, various tour agencies in town will take you out there in a bus, park for an hour, then bring you back. No guarantees or anything of course, but if gives you a chance to see the lava. Staying at the lava-view hotels is a good idea because that way you can just hang out in your room, or at dinner, for as long as you want while watching for the clouds to clear away.