I had a treasure hunt at my DD party last year.
I hid :
Ghost lollies - Put a tissue over a lolly, hold it in place with a bit of wool and draw a ghostly face onto the tissue, you now have your ghost.
Satsuma/Tangerine Pumpkin head - Using a black felt tip pen draw a pumpkin face on one of the small oranges with a zig zag aroung the top so it looks like a lid.
Other Halloween sweets, plastic spiders etc.
Each child was given a Trick or Treat bag and told what they needed to find (one each).
We have played variations of this game at every party my daughter has had since she was 3 years old. You just hide fewer things in easier places for younger children. The only thing I would say thought is keep a record of everything you hide so that you can find that last missing lolly.
We also made Spider Biscuits:
You need Rich Tea/Digestive Biscuits (two biscuits per spider), Marshmallows, Icing (Various Colours), mleted chocolate, Shoelace Liquorice (you need 4 pieces for each spider), Jelly Tots, Smarties.....
Put one marshmallow onto one biscuit and put in microwave for 10 seconds. The marshmallow will swell up, take the biscuit out of microwave and quickly lay the four piece of liquorice over the marshmallow to make the legs. Place the other biscuit on top and gently squeeze. This can be done before the get children arrive.
Then let the children decorate the biscuits using the icing, chocolate and sweets.
You will need a cover for your table or floor and possible something to protect the children. I normally have one friend who overseas this and the children do it when they do not want to play the games.
Another game I always like is "Sleeping Ghosts" (Sleeping Lions). I normally play this towards the end of the party to help settle the kids down before sending them back to their parents. It also gives me a little bit of a break. One time my DD actually feel a sleep.
I also read everyone a story whilst waiting for parent sto turn up. For the Halloween we read "Room on my Broom" and "The Gruffalo". Even at 5 years old the children sat and listened and joined in with "The Gruffalo".