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Larger than the Tiscapa Lagoon, the Asososca Lagoon is also fenced in, and closed to the public; apparently it's part of the city's water supply. As of 2016, the water company has been replacing parts of the old dilapidated fence. The lagoon can be seen from certain vantage points nearby, and its near-vertical walls are rather stunning.
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Museo Sitio Huellas de Acahualinca
Footprints of a group of around 10 people that walked towards the lake 6000 years ago. The tracks were found 4 meters below the surface and were preserved thanks to a nearby volcano eruption. Note that this museum is located in a rough neighbourhood. Taking a taxi is highly recommended.
Estadio Nacional Dennis Martinez
While you might be surprised to hear it, baseball is as much a national pastime in Nicaragua as it is in the US, maybe even more so. The national stadium (named for Major League pitcher Dennis Martinez) has space for 15,000 people, making it bigger than the national soccer stadium. The stadium opened in 2017. Besides baseball games of the national team it is mostly used for home games of the "Indios del Boér", Managua's home team and the serial champion of the Nicaraguan league. Parking available.
A diminutive arboretum, nice and shady.
The plaza-sized hilltop viewing platform on the northern lip of the crater of the Laguna de Tiscapa, known as Loma de Tiscapa, used to be the site of the presidential palace and the headquarters of the National Guard in the Somoza era (until they were destroyed in the earthquake of 1972); political prisoners were tortured in a prison close by. This is where you'll also find the landmark silhouette of Augusto C Sandino (with a little American tanquette next to it), as well as military memorials. The hill offers the best views in town, both to the downtown and Lake Managua to the north, and to the Carretera a Masaya area to the south. The Nicaraguan Army headquarters are located on the hill's northern slope, and are definitely not open to the public, though pedestrians are allowed on the street straight north from the hill, which is lined with large interpretive signs in Spanish about the military history of Nicaragua (from the Sandinista point of view, of course).
Fresh water lagoon in the crater of an extinct volcano. As of 2016, the lake and its wooded shores, with some paths and stairs, were fenced off and were off limits to visitors. However, there's a good overview of it from the Loma de Tiscapa Park just north of the lagoon (see below).