I'm thinking about holding some kind of contest but I don't know where you would do that?

depends on the type of contest. the human avatars, art discussion, writing discussion, and profiles & graphics discussion each have their own contest subforums.

Uhh.. If I had to describe it.. sweats Scavenger Hunt? I want to do like "cooking" contests

in the Rules for Subeta (it's the middle of the links at the bottom of every page, they're tiny) don't specify where you can have Contests. I would look in each Forum and look and see if they have forbidden contests unless they're in that subsection. If not, I'd put it where it seems likely. My guess either Games or General,

You can File a Ticket in Help, or ask the MiniMod of the Forum or Subforum you want to have it on and see what they say.