Masaya Volcano National Park, about 30 kilometres south of Managua, is one of the most accessible active volcanoes in the world. You can drive almost to the crater rim of the Santiago vent and walk to the edge and look directly down into the lava lake churning below — a genuinely spectacular and slightly terrifying experience that requires no hiking and very little effort.
The park covers two volcanoes and five craters; the active Santiago crater is the main attraction but the park also has a colonial-era fortress built by the Spanish to contain the "infernal" activity, a museum of Nicaraguan geology and pre-Columbian culture, and trails through the surrounding tropical forest. Night visits (when the lava glow is most dramatic) can be arranged through tour operators in Managua or Granada. Combined with a visit to Granada and the Masaya market town, this is the best day trip from Managua.