Isla de Ometepe is a large island in Lake Nicaragua formed by two conjoined volcanoes — Concepción (active, 1,610m) and Maderas (dormant, 1,394m) — linked by a fertile isthmus. It is one of the most beautiful places in Nicaragua and deserves more than a day trip, but even a brief visit from Managua via Granada is worthwhile.
The ferry from San Jorge (near Rivas) takes about an hour; buses from Managua to San Jorge take about two hours, making the whole trip feasible as a long day or better as an overnight. On the island you can hike both volcanoes (Maderas has a crater lake at the top), visit pre-Columbian petroglyphs, swim in freshwater, and rent a bicycle to explore the isthmus. The local communities are welcoming and the pace is very different from the capital.