Nicaragua on Foot?
I tripped on a site called La Paz on Foot. I offer it as an example of something that could make sense in at least some parts of Nicaragua. While they offer longer treks, their bread and butter seems to be 4 and 7 hour treks in the La Paz area.
That type of trek would probably only work in Managua but that's a good start. Their treks are with a small group (it could be just one person) with a bi-lingual guide. From reading the description, it sounds like a great chance to learn about/see things the typical tourist either would not see or have fears/concerns about seeing.
If it was a success, doing at least half-day treks in other areas such as Granada, Estelí and Matagalpa would seem like a good next step.