Plumbuita Monastery sits north of central Bucharest near the lakes, away from the city's usual museum-and-boulevard circuit. Its walls and church preserve the atmosphere of an older, semi-rural Bucharest, when monasteries were also fortified compounds and landowners in their own right.
It is not as immediately showy as Stavropoleos, but that is the point. Come for carved stone, painted interiors, and a quieter Orthodox setting where the modern city drops back for a while. It works best as a deliberate detour, especially if you are already exploring the northern parks and lake districts.