There is a short (half-hour) trail through the coffee farm at the bottom of Volcán Mombacho , where you wait for your ride up. Once on top, there are two main trails to choose from: Sendero el Cráter, which encircles the forest-lined crater, and features a moss-lined tunnel, several lookouts, and a spur trail to the fumaroles (holes in the ground venting hot sulfurous air).
The fumaroles area is an open, grassy part of the volcano with blazing wildflowers and an incredible view of Granada  and the isletas. The whole loop, including the spur, is 1.5 kilometers, with a few ups and downs, and takes a casual hour to walk.
The Sendero la Puma is considerably more challenging—a four-kilometer loop with several difficult climbs that lead to breathtaking viewpoints. It begins at a turnoff from the fumaroles trail, and you should allow a minimum of three hours to complete it (and lots of water).
Well-trained, knowledgeable local guides (some with English) are available for $5 (per group, plus tip) for the Sendero el Cráter and $10 for the Sendero la Puma. Note, because of the altitude and the clouds, the visibility from these trails may be much diminished on bad-weather days.
The Volcán Mombacho reserve  is closed on Mondays for maintenance, and usually restricts Tuesdays and Wednesdays to organized groups. From Thursday to Sunday, all are welcome. The entrance fee ($7.50 for foreigners, $5 for Nicas and residents, $4 for students and children) includes admission to the reserve, transport to and from the top of the volcano, and insurance.
If you’ve got the time and the strong legs, feel free to hike all the way up the steep road yourself. Allow a couple of hours (and lots of water) to reach the top.