If you happen to have a couple witches hats left over from previous years' costumes, use those! I couldn't locate my Halloween decor and costumes so I decided to make some out of construction paper.
I needed them big enough to hide a decent sized "treat" underneath so I taped two pieces of construction paper together, the red one on the longer edge and the black one on the shorter.
I cut a bit off the top of the black, then clipped the corner. I also cut a strip off the red to use on the brim.
I used a bit of the black scrape on the back and taped it up in a cone. I also had to trim down the edges of the red to make it sit flat. For the brim I taped the red strip I cut off earlier to a piece of black paper, traced a circle using a paper plate ad cut it out.
I made two more in different colors (see my candy corn themed hat?).
All three made in the same simple fashion. I would suggest doing one for each child involved in the hunt, and 1 or 2 extra, to leave empty underneath or hide a 'trick' like a baggy full of broccoli, or a bit of trash like a toilet paper tube.
Here's the prize hidden under one of the hats, a Halloween cup with a straw.