Here are some budget friendly ways to make a generic headless mannequin more interesting to attract the attention of passerby.
You can purchase various props/supplies from places like Oriental Trading. Or even a home goods store that sells unique products like this set of lips which are an over sized pillow.
You can make oversized hearts out of foam core to use like this for Valentine’s day.
Or turn a headless mannequin upside down and put it in a bucket, pot or bag. 
This wedding display was to represent “Time to Get Married”
You can also over the necks of the mannequins with products you are selling in your store, like this display from Zara with purses as heads.
For more ideas like this, check out this Pinterest board on Heads for Headless mannequins.
