It is incredibly important to have a prize at the end of a treasure hunt, although the size of the prize does not always matter.
Having a prize draws the hunt to an end and means that the hunters know they have succeeded in their quest. It can be awkward if they do not realise that they have reached the end of the treasure hunt and continue to look for clues.
What makes the best prize can vary from hunt to hunt. Easter treasure hunts often lead to Easter eggs. At Christmas the clues may lead to a small present that can be opened before main gifts. The important thing is that the prize can bes hared between everyone, which is why chocolate or similar is a good choice. If the prize is edible or delicate, wrap it up well if hidden outside. If the intended prize is too big to hide, or isn't even a physical thing, the last clue could read something like "Congratulations, you've won! To find out your prize you need to tell Mum the password: chocolate cheese".
Keep the prize simple and something you know the hunters will like. Remember, half the fun is solving the clues!!