Cocibolca is the enormous lake that looks far too big for the tiny Central American isthmus. It was considered for a time as a suitable place to cut the isthmus in half with a canal; this is what first drew the unfortunate attention of the US to the area.
Guidebooks always mention that Cocibolca is home to the only freshwater sharks in the world. This is true, but they are very rare now; shark-finning during the Somoza years killed off tens of thousands of sharks in the lake and in the Rio San Juan that connects the lake to the Caribbean. According to some Nicas the practice continues in barbaric Costa Rica.
Granada and San Jorge (just outside Rivas) are the two main access points for the lake; both run ferries to Ometepe. Close to Granada are hundreds (allegedly one for every day of the year) of small islands home to some very private villas, an old fort and the odd tourist restaurant.
Ometepe is truly Nicaraguan; this means it has the highest concentrations of insects, gallo pinto and machetes in the country. It also has two very different volcanoes each garbed in clouds, and some lacklustre beaches. A good place to see wildlife, and to pick at your mosquito bites while waiting for the rain to stop and the fireflies to come out.
In the southeast corner of the lake is the Solentiname Archipelago, home to an odd artist colony founded by a priest turned Sandinista (Pope John Paul II berated the shit out of him when he visited).