The idea has evolved somewhat so I'm happy to share!
DS has asked to do one on our road instead of in the woods. So we are going to do a photo scavenger hunt as DH can borrow some flip cameras from work. We live on a relatively traffic-free cul-de-sac that all the kids will know fairly well as it's a well-used cut through on the way to school. It also provides a very good boundary as everything will be able to be found on the road and not beyond the road sign.
Examples of things to take photos of will be:
a German car
a number (which will be the result of a sum ie 2x8)
a picture of a bike
a bush with red berries
a crack in the pavement
someone's mum (some of the kids live on the road too)
seven feet
someone (not in the team) making a funny face
The party is now a treasure hunt party as DS wants to dress as a pirate. So we are going to hide chocolate coins at various locations on the hunt ie under the bush with red berries, at the bottom of the sign for the cycle path (there are two so only one will have coins at the bottom). Whichever team gets there first wins the coins too for bonus points.
We are starting at 4pm and the sunsets at 4.30pm so their deadline will be to take as many pictures as they can before it gets dark. We will include some funny things in there as the plan is that when it's dark, we go inside and look at all the pictures on the TV. So it will be entertaining to see photos of people making funny faces, doing a handstand etc.
Each team will have a grown up with them so they won't be walking around on their own and approaching strangers. And they can help with directing the children towards the coins etc if required.
We have Riddle Me which we will use to put together an indoor treasure hunt with clues, ie "You draw these when it's bedtime" - then you have to go to the curtains to find the next clue. This will have to be our plan B if it's raining heavily, though DS wants to do that as well in the dark with torches. 
We didn't want to do a scavenger hunt indoors as we felt that DS had an advantage and knows where to find everything.
We are also going to have a pictionary game up our sleeve as I find this is a really good way of filling the time at the end of the party if you have run out of things to do. You can just prop an A3 pad up on a table and the birthday child goes first. Whoever guesses it right, gets the next go. It's simple and they love it!