Managua is more useful as a base than as a destination, and almost everything worth seeing is within two hours of it.
Granada, the colonial city on Lake Nicaragua, is an hour south and the most common day out; Masaya is closer still, with a craft market and a volcano you can drive to the rim of and look down into a live lava lake — one of very few places on earth where that is possible from a car park.
Northwest, León is about ninety minutes, with the ruins of León Viejo and the cone of Momotombo on the way, and Cerro Negro beyond it for the volcano boarding.
Laguna de Apoyo, a crater lake with clear warm water, is forty minutes away and the best swim near the capital. Isla Ometepe is possible in a day but deserves two or three.