You will never be at a loss for a hiking trail in either country, and somewhere close to your hotel, there is most likely a dirt track with a waterfall at the end of it. Juayúa, on El Salvador's Ruta de las Flores, is one such place, with lovely walks over the hills, around lakes, and through coffee plantations. Parque Nacional El Imposible is imposible to miss and easily the most accessible primary forest in the country. The popular Parque Nacional Los Volcanes has volcanoes to climb, including the dark, barren cone known as Volcán Izalco, while the eastern town of Perquín has trails that were originally created by guerrillas during the war years.
Matagalpa in the Nicaraguan highlands has a dizzying array of hikes, as does Reserva Natural Miraflor near Estelí. However, what makes Nicaragua special is the range of volcanoes begging to be climbed there, including the twin peaks of Isla de Ometepe. Be sure to hire a guide to show you the way around these often-smoking, active peaks.