The Museo Carlos Fonseca Amador occupies the house where the FSLN's founder was born in 1936. It is a small but earnest museum that documents Fonseca's life, the founding of the Sandinista movement, and the 1979 revolution from a perspective that is unambiguously sympathetic to the revolutionary cause.
Even if you're skeptical of hagiography, the museum is worth a visit as a window into how the Sandinistas understand their own history. The exhibits include photographs, personal effects, and documents from the revolutionary period. It's also simply interesting to stand in the house where one of Central America's most significant revolutionary figures grew up.