Pacaya is an active volcano that is currently continuously erupting.
Pacaya first erupted approximately 23,000 years ago and has erupted at least 23 times since the Spanish conquest of Guatemala. After being dormant for a century, it erupted violently in 1965 and has been erupting continuously since then, sometimes showering the nearby city of Antigua Guatemala with ash.
This is an active volcano where you can get close enough to the lava flows to poke sticks in it. The ground is made up of loose volcanic rock and is quite unstable in parts.
Flora and fauna
To get there independently from Guatemala City, take TransMetro line 12 to the last station at Centra Sur (known as the CENMA), then get on a camioneta (chicken bus) bound for Escuintla, and ask to get off at the cruce to San Vicente. It should cost 5Q. From the cruce, take a San Vicente camioneta, and ask if they are going all the way to San Francisco (4Q). If not, switch to a San Francisco camioneta at the entrance to San Vicente. Either get dropped off at the Calderas entrance or in San Francisco. Either is fine.
Take a bus from Antigua Guatemala. I believe it would be safe to do this trip on your own although many people state otherwise so you might want to go with a tour company. Shutlles from Antigua to Pacaya volcano everyday the cost is around $15 US per person.
There is a park entrance fee of Q50 Quetzals.
Walk and take a hiking stick the ground is rocky and in sections it is unstable. You should join a tour because this is an active volcano, and in general it it not advisable to visit volcanoes without a guide in Guatemala.
Molten rock, lava flows.
Take photos, cook toast or marshmallows.
Hiking Stick - it'll cost you less than $1.00 and should be returned to the local children that sold it to you.
There are shops at the base of the trail.
You can camp here.