A few years ago, at least in the Qld community, it was something that was found or given to you that could be hung from the tow hook. If we saw anyone with something they bought specifically to put on the hook they were flamed. I found a tiny surfboard in the gutter outside Revhead's house and it was on there until it broke. I now have a Dora doll that my 3 year old daughter insisted go on there after she saw others at last years SAU Charity Cruise.
Yes, it is is kinda silly, but who cares really??? And I won't disappoint my daughter...