Halloween is a big weekend for me every year. On the Saturday closest to Halloween (29th this year), Roppongi is rich with costumed bar-hoppers after 10pm. Many of the foreigners wear thoughtfully crafted costumes whereas Japanese drinkers, who have realized that most people are in costumes, crowd the Don Quixote store buying the last of the novelty costumes. Many bars offer a gift or free drink to those who wear elaborate costumes.
On the Sunday closest to Halloween (30th this year), Kawasaki has a huge Halloween parade. It is bigger than events I've seen in the states and the attendees are very serious about their costumes. The homepage is here:
ラ チッタデッラ【川崎】～KAWASAKI Halloween 2011
If you want to be in the parade or enter the contest, you have to register on the website. Register early as they reach capacity quite early in October. Here is a video that I recorded at last year's parade (I am the guy in Iron Maiden's Eddie costume at the beginning and throughout the video):
Japanese Halloween Parade - YouTube
Metropolis has a Halloween Glitterball on a weekday before Halloween weekend (This year on Thursday 27th), but who can really get out on a weekday? Somehow it's still a very crowded event. This is the online ticket sales:
Metropolis Glitterball 2011 - The Monster Bash
Finally, there is a regular ghost tour in Tokyo. They usually do a cemetery walk on the Saturday night closest to Halloween. It is a good tour, but it is full of creepy facts or murderous stories rather than walking into a possibly haunted house. In other words, it's more of an interesting dark history tour. It has great reviews.
Haunted Tokyo Tours
The attached photo was taken at an early evening hour in Roppongi. It gives you an idea of the crowd. I am Michael in this photo.