Described by the Spaniards as the gates of hell, the craters that comprise Volcán Masaya National Park are the most easily accessible active volcanoes in the country. The two volcanoes at the park, Masaya and Nindirí, together comprise five craters. Of these, Cráter Santiago is still very active and bubbling with red-hot lava. The park entrance is just 7km from Masaya on the Managua highway and most tour operators in Granada run evening trips to the crater.
At the time of writing, visitors were only allowed to access the Plaza de Oviedo, a clearing by the Santiago crater's rim named after the 16th-century Spanish monk who, suspecting that the bubbling lava was gold, descended to the crater with a bag and small shovel – and came back alive. Here, the smell of sulfur is strong, and you only get 15 minutes at the viewpoint – enough to watch the molten magma play – before you're ushered back into your vehicle.
The park has several marked hiking trails, many of which require a guide (prices vary). These include the lava tunnels of Tzinancanostoc and El Comalito, a small, steam-emitting volcanic cone. They were closed at the time of writing, but may reopen in future.
From the summit of Volcán Masaya (632m), the easternmost volcano, you get a wonderful view of the surrounding countryside, including the Laguna de Masaya and town of Masaya.