Arenal 1968 Volcano Trail
Traverse through lava fields formed after the great Arenal Volcano eruption of 1968. We recommend taking a longer 3 miles route, "The Bosque 1968 Trail," that, in addition to the iconic Lava Flow summit viewpoint of Arenal Volcano, features Duck Lake, formed when the eruptions took place (check the AllTrails link).
The hike is quite strenuous since the terrain has some steep portions and a lot of roots and rocks. Make sure to take your hiking shoes and keep in mind that it could rain if you arrive after lunch. In general, it's a good idea to keep your rain jacket and mosquito repellent in your backpack on hikes in Costa Rica.