This is not to say that a party treasure hunt doesn't require a lot of work to organize! As with any theme party, a party treasure hunt needs invitations, a treasure map and of course the treasure! Not to mention food, games and maybe even costumes.
First a bit of history. There are different types of treasure hunt. One form of party treasure hunt is actually based on a Scandinavian activity called "Orienteering". Orienteering has less to with treasure and more to do with map reading. The object is to find a flag or some specific object that is hidden in the woods. The kids then use a compass and map to track coordinates and ultimately find the treasure. I have always found this a little to complicated to maintain a child's interest. So I modify things and still use a map, but it's more like going from one clue to the other without the need of a compass or anything else. Trust me it's still fun.
Starting with the invitations, consider preparing a "message in a bottle". In this case it will be messages, because you'll need a different bottle for each invitation. Try using "mini" 8 ounce Coke bottles and put your "message" inside. This can be a treasure map with directions to your house or some kind of written invitation with a skull and cross bones on it.
Some games can include a pirate coin "penny toss" into a pail of water or into small glasses. A tug of war can be fun also. But don't lose sight of why everyone is at the party treasure hunt: they want to find some treasure! If the party treasure hunt is at the park or beach, you need to be there way ahead of time to hide all the different items. And all these items need to correspond to a treasure map.
You may want to consider dividing everyone up into teams of 3 or 4. To keep them busy, each team needs to find a minimum of 20 items. If you throw a treasure hunt party and you want it to be a success, it's important to have lots of treasure on hand. Either items you have planted ahead of time, or give each team a list of items that know a head of time they can find. And if you have a time limit, say an hour or even less that just adds to the excitement.
Another idea for the treasure hunt is to use string. That's correct, I said string. A string treasure hunt can be pretty complicated because you need to lay one very long piece of string and trail it everywhere! At the end of this very long piece of string will be the treasure. Or at intervals along the piece of string you'll have different treasures to find.
Before the treasure hunt begins, gather the kids around to read a story about pirates and buried treasure. This gets everyone in the mood to hunt treasure, and it will make your party treasure hunt a memorable one. If you really want to throw a treasure hunt party that the kids will be talking about for weeks, have them all dress up as pirates! You're guaranteed to see the kids all show up with patches over their eyes and pistols in the pockets. Not to mention colored robes and shirts!
Now let's not confuse a treasure hunt with a scavenger hunt. They are both similar, but I like to think of the scavenger hunt as a treasure hunt's "distant cousin". Typically a scavenger hunt has the teams go out and find run-of- the-mill items like a tire, a cup, a cigarette and the like. The treasure hunt should have specific items to be found that can be considered treasure. That's why earlier it as mentioned that the items need to be planted ahead of time.
However you decide to do it, keep it focused on pirates and treasure and you'll do fine.