Here are The Traveling Professor's picks for places to spend Halloween in Paris:

Père Lachaise Cemetery (Cimetière du Père-Lachaise)
Métro:  Père Lachaise or Philippe Auguste.   Bus: 69, 76, 96.
Museum Pass:  No Admission Fee.
This famous cemetery is a “Who’s Who” of legendary French and foreign writers, musicians, authors, politicians, and artists.  Jim Morrison, Max Ernst, Maria Callas, Oscar Wilde, Edith Piaf, Colette, Gertrude Stein, and many other recognized names are memorialized here.  Don’t overlook the poignant holocaust monuments.  Buy the map at the entrance close to the “Père Lachaise” métro station. There are restrooms near the visitor center.  Bear in mind that the cemetery is large and may take some time to cover.  Wear comfortable shoes, the walkways are cobblestone and the terrain can be hilly.  The website is excellent, describing the location of the graves: www.pere-lachaise.com.

Catacombs (Les Catacombs) 
Métro:  Denfert-Rochereau. Bus:  38, 68.
Museum Pass: No – admission is €8.  Closed: Monday   
This is an underground quarry holding about six million skeletons, relocated from other burial grounds in Paris.  The Germans used it as a bunker and the French Resistance made use of the tunnel system during WW II. It is fascinating although some find it gruesome and/or claustrophobic. I think it is a lot of fun.  Bring a flashlight and watch your head.  It is 138 steps down to the catacombs, 83 back up.  About one hour.  Website:  www.catacombes.paris.fr.