Rivas is the town on the isthmus between Lake Nicaragua and the Pacific, and almost every traveller in southern Nicaragua passes through it without stopping: the ferry to Isla Ometepe leaves from San Jorge five minutes away, San Juan del Sur is half an hour west, and the Costa Rican border at Peñas Blancas is forty minutes south.
If you are stuck here, the church on the central park has a dome painted with a naval battle in which Catholic ships defeat Protestant ones, which is not something you see every day, and the market is a good one.