Mancarrón's greatest human-made gift is Nuestra Señora de Solentiname, where populist priest Ernesto Cardenal ran a rather enlightened parish. Constructed by the community in 1979, it features a beautiful whitewashed nave from within which you can still hear the lake and feel the trees. The altar spares the usual golden idolatry and instead is graced with a colorful yet humble mural depicting life in the archipelago. If there's no one around, ask up the hill at the museum.
Nuestra Señora de Solentiname
Top choice in The Río San Juan & Islas Solentiname