Arenal rises 5436 feet above sea level, and is the most active of the volcanoes in Costa Rica. There are various tour companies which offer night hikes to various lookout points in the area. It is best to do this on a clear, non-rainy day, which is a 30% chance. An alternative, if you rented a car, is to drive around close to the volcano and look for the area where tour buses stop to let their customers see the volcano at night. This was exactly what we did. At various times we saw ash columns, lava flows, and sudden explosions if the volcano is not covered by clouds. In fact, just as we stopped we were treated to the volcano erupting. The embers burned for 2 minutes. It looked like the orange top of a triangular shaped incense when burned. After that, we saw some minor eruptions. The organized tours do not allow BYOB. However, if you rent a car you can drink to pass the time between eruptions. Trust me it can be a long, long wait between eruptions. Drink some of the local Rock Ice, which is a clear, alcoholic, lemony drink in a can with salt already in it.
Be aware that because of low light conditions your digital camera may not catch the eruption. To snap a digital photo properly, you need a tripod and have to remember to set your camera for night or low light conditions.