St. Denis (pronounced "San Denn-EE") is pretty cool. That's where all the king's and queens were buried...until the revolutionaries took their bodies out to a field. But their tombs are interesting. Go during the day and get back to Paris during the day. It's not the nicest area, but worth seeing.
Montmartre has some pretty eerie history as well, worth checking out and taking a break to sit at a cafe.
You can go see all the mass-buried bones under the Seine. That's nice and creepy.
Oh! And the Pantheon is interesting, too. Roman ruins in that area as well. almost 8 years ago