The hike up the mountain trail to the "active" crater is gentle enough for anyone to manage. If the weather is not clear you may not be able to see it however. The crater is basically a sulfur lake now and you can smell the chemicals in the air, if the wind happens to be coming in your direction.
It's a beautiful site to visit if you are staying in the San Jose or Alajuela area it's a short drive and pretty much every tour agency offering will contain at least a stop at this location.
The 2nd crater referred to as the Lagoon is a stiff hike up a carefully groomed path so be prepared to break a sweat or two there.
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