Pacaya is definitely worth a visit. Lots of tours on offer - we asked our hotel for a recommendation, and paid $10. Pickup at the hotel at 6 a.m., coffee stop (thank goodness) and we were at the park by about 7:30. Our tour included a helpful guide, though only Spanish-speaking. There is also a $6 park admission fee.
The hike in isn't bad - you are climbing a volcano, but the pace isn't rapid, they are viewpoints where you can rest and it's probably about a 90-minute walk. People with horses will offer you a ride - about $10 roundtrip, or $5 one-way seemed to be the norm. That's an option. (They will also follow you up in case you change your mind, sometimes irritatingly close.)
The hike takes you to the lava field from the 2010 eruption, and it's eerily beautiful and surreal - sprawling black rock, jagged spikes, the peak and everywhere steam and smoke streaming from vents. The guide takes out to a hotspot where you can toast marshmallows over the heat coming out of a vent.
Back in town by lunchtime.
The morning was definitely the best time in October - clouds rolled in by the afternoon, limiting views.
Highly recommended, as an experience unlike any I've had.
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