The San Martin Pinario Church
San Martin Pinario is famous in Santiago for its for its lodging in Santiago. Great location, just next to the Cathedral.
But one place I would recommend to visit is the church that is located in the same complex (San Martin Pinario is huge). The entrance to the church is from the side, but once inside the church is spectacular.
There is a small entrance fee, but I would not be surprised if you have the whole church to yourself (this is a hidden gem).