This year, Mainstreet is converting two storeys of Wakefield Chambers, on the corner of Ridgway St and Victoria Ave, into a haunted house.
The building has been vacated due to earthquake risk.
Remaining tenant More FM will be playing freaky music and AmDram actors-in-training will be the frighteners.
The museum will have its own haunted area, and will screen a D-grade horror movie for young people to ad lib the voices to.
There will be a total of 20 places downtown supplied with sweets from Whanganui Confectionery 'n' Snax, where children can trick or treat.
A printed flyer will list the 20 venues. Mr Follett said two of them had occupants who disliked the "dark" side of Halloween.
They would be "doing a light spin on it".
One of the venues is the museum, where young people from Cheryl Amos' Rainbow River Theatre will be planted among the exhibits in spooky costumes.
Retailers and occupants of the venues will decide for themselves how to give out the treats.
"Some might insist the kids do something for them."
Children under 5 will be the only ones able to trick or treat from 3pm to 3.30pm. After that it will be open slather until 7pm.
There was a great response to the museum's Not So Scary Open Mic Fright Night last year, said external relations officer Kyle Dalton.
Organisers chose a C or D-grade movie, "the worse the better, so that they're funny".
Then they turned down the sound at times and passed around two microphones so the teenagers watching could ad lib the dialogue.
Last year, there were 150 in the Davis Lecture Theatre.
"In the end, they were pretty much ad libbing the whole thing, once they got into the spirit of things and realised there was no wrong or right in it," Mr Dalton said.
This year's film hasn't been chosen yet. It runs from 7.30pm to 9pm and entry will cost $2.
