Bad promotion ideas start with 'wouldn't it be cool if…'. Good ones start with 'which customer did what, and how do we get more of them to do it again?'
Once the behaviour is defined, the mechanic almost picks itself: threshold rewards for basket size, prediction mechanics for engagement, sports challenges for PR moments.
The prize is the last decision, not the first. Sized correctly against the target behaviour, it becomes the campaign — sized incorrectly, it becomes the invoice.