Most people visit Volcan Poaz on a guided tour from San Jose. Since every hotel and hostel in the country will book a tour to Poaz, there's little reason to list any of the tour companies!
Volcan Poaz can also be visited independently. Public transport is available to the crater daily. A bus leaves (about $3 one way) from Avenida 2, Calle 11 and Calle 12 at 8:30AM. The bus returns at 2:30PM.
Butterfly seen on the slopes of Volcan Poaz.
The hike is very easy. From the visitor center, simply follow the paved trail through the lush forest for 0.5 km to the crater rim. There are many interesting plants with huge leaves along the way. From there, you will have an incredible view into the 300 meter deep/1.5 km wide crater. The crater walls are very colorful as is the blue-green lake (colored by copper ore and sulphur). Hopefully the weather will be clear on your visit!
Rain gear, a jacket, and a camera. That's about it!