St. Denis is amazing, especially if you have ever had an interest in French history, or even history in general. I went there a little while back and the first 20 minutes of my holiday video were of the inside of this place.
St. Denis himself was the first bishop of Paris. He became closely linked with the monarchy. The Abbey was founded here in the 7th century, over the place where the saint was buried. The Abbey we see today was built between 1137 and 1281 and was begun by Abbot Suger. The church is well known as a royal burial church. The first king to be buried there was Clovis I in 511. Between the 10th century and the 19th century, only three kings of France were not buried in the church. During the French Revolution the tombs were opened and the bodies were flung into a nearby pit. The tombs themselves were only saved because they were claimed by a museum. When the monarchy made its brief return, the bodies of Louis XVI and Marie Antoinette were found and added to the abbey. Altogether there are 42 kings, 32 queens and 63 other royal burials in the abbey. The tombs were restored to the abbey after the Revolution.
The Abbey is not in the heart of Paris. It is located well outside the centre (3km) in an area which is far from pretty. The metro line 13 will take you there (stop at Basilique de Saint Denis) as will the RER line D. The entrance fee is 6.5 Euros and audioguides cost another 4 Euros (6 Euros for 2 people). In the summer the Abbey opens from 10-6.15 (from 1pm on Sundays) and in the winter it opens 10-5.15 (from 1pm on Sundays).
Ok, so the area isn't great but the Abbey more than makes up for it. It is very gothic looking. Inside you can see all the tombs and you can get right up close to them - no ropes! I also love the fact that you can film and take photos. I spent about 1.5-2 hours looking round, there really is a lot to see. Even if tombs are not your thing then it is worth a visit for the vastness of the inside, the architecture and the overall beauty. A couple of things to look out for include: the tomb of Louis XVI and Marie Antoinette - very poignant; the heart (yes literally, in a small glass tomb) of Louis XVII (he died during the Revolution when he was about 10 - very strange to look at); some of the cadaver tombs, which show the naked bodies of those they represent, as well as them fully clothed.
It may seem like a bit of a hassle to go out to St. Denis but if you have enough time it is well worth the visit!