Ometepe, an Island in the middle of Lake Nicaragua near Rivas is a perfect destination for hiking, nature trails and relaxing on the many lake beaches. You can summit 2 of the tallest volcanoes in Nicaragua, one in a rain forest and the other above the tree line. There are many small hotels and hostels as well as mid and upper tier hotels. A must stop for lunch or dinner is El Campestre, a small roadside restaurant on the east side of the south island (Maderas). They have an extensive menu, the food is locally grown, good quality and very tasty.
Catch the ferry from San Jorge (Rivas). If you have a vehicle, a reservation is recommended (your hotel can make one for you) or just show up and walk on. The fee is only a few dollars to walk on and not much more for a vehicle but most of the ferries can only carry 6-9 vehicles, hence the reservation requirement.