You know how they have suckling pig at luau right, instead have suckling people... or person.
Tiki's are pretty bad ass in their own right... Big Lots had several Tiki products available a month or two ago - I haven't been since, but they were cheap and great (for both regular and haunted luau)
Pigmy's -- little guys with those huge masks.... I see cardboard and doll legs/hand with a straight stick for a spear -- you can pump out 10 of those guys for next to nothing.
As far as ambiance, palm fronds can take up a lot of space and make empty, blank areas 'full'. A looping sound (MP3 player or CD) of deep drums playing in certain areas. Coconuts tied together strung up all over. Hatching (like you'd see on a roof) place in certain areas -- those are the type of things that can fill in unused space or areas that might take away from your theme. Lighting can be a big help with this theme... yellows, reds, oranges and even some green lights place in corners of rooms, behind trees or bushes... that stuff adds to the feel without getting too involved (both time and money). If you've got a fog machine, I'd use it.
Black leis for everyone? Black drink umbrellas. I don't even know if they make those.
What kind of food are you having? I picture some meat (pork especially)... depending on how you prepare it, have some mini blow darts sticking out. That could be as simple as pencils with frilly red, feathery crap on the top (dollar stores always have that sort of thing).
Shrunken heads on stakes -- made from doll heads, painted, burned, and hair tied with some rope.
And of course, Tiki torches (just be careful of placement if you're going to have guests with costumes walking by them).
Instead of carving pumpkins - carve watermelon, pineapple, or any tropical fruit. Light 'em up inside with candles too!
It's late and that's everything off the top of my head. Maybe I'll actually search online for more ideas, but later!
I think all the ideas I suggested are able to be created or bought/created this time of year at most stores. And everything that I had in my head when I was typing all this was fairly inexpensive, but could be turned into something awesome. Keep us updated.
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