Great place to visit volcanoes. The Arenal Volcano is the only volcano in Costa Rica constantly active since it woke up after 400 years with the big 1968 eruption producing huge ash columns, explosions & glowing red lava almost every day.
One of the greatest things in Arenal is the Tabacon hot springs. Nestled in the rain forest is a little oasis complete with hot baths, massages, and great food. It's a must.
Many people choose to stay at the Tabacon resort when visiting Arenal. While this is convenient to the spa, there are many other places nearby that offer similar quality rooms for much less.