This is an incredible site. I've been twice to St. Denis, and tried to take my wife a third time, but it was New Year's Day and the cathedral was closed. 

Getting off the Metro, the area is a bit depressed, but the people are hard scrabble working folks and immigrants, lots of street vendors.

It's been a long time since my visit, but I remember the Gothic history of the place being quite remarkable, and the tombs of all the French royalty dating back to Clovis, Dagobert, Pepin the Short, and up to Marie Antoinette - are you kidding me? Then, you learn the Parisians during the revolution pulled the corpses and bones into the street having strewn the bodies about.

They were re interred together in the crypt by Napoleon. The church itself is one of the earliest if not the earliest Gothic cathedrals. A definite highlight which should be included on the list. 

Totally worth a stop and a couple hours time to visit.