Lac Sacre Mangatsa is a small sacred lake inland from Mahajanga, wrapped in local custom and mangrove-country quiet. It is the kind of excursion that makes sense if you have already seen the waterfront and want a gentler look at the Boeny hinterland.
Go with someone local, both for practical navigation and for etiquette. Sacred sites in Madagascar are not museum exhibits; some have taboos about behaviour, food, clothing, or photography, and it is much easier to get it right if you ask before you act.