I've always had a treasure hunt for my kid's Birthday Parties. Here's some ideas I came up with.
Before my kids could read I'd give them stepping stones, footprints, or arrows to follow. I'd give them balloons and feet to decorate. If you use heavy enough cardboard, these creatures always land on their feet. You can also use disposable gloves, like I did here.
Another activity I liked to do with them was a balloon war- you split the children into teams, give each team an equal amount of balloons, face them off across a rope or a line and see which team can get all their balloons on the other side first. Kids love this. Pass the water balloons can also be fun, as long as the parents don't mind their kids getting their clothes wet.
Kids of all ages like the balloon games.
When the kids got older I would give them clues to follow. The hunt would usually end with UFO's.
To make them, you just have to blue tac two plastic bowls together. Inside, there would be three dimensional fingernail stick-ons. You can find them in any supermarket. The children would use these to decorate the outside of it.
You also put in the party candy of course, ideally with a little finger puppet alien, or a do it yourself alien made of a blown up water balloon and a cut out set of feet. You'd get the kids to draw a face on it.
I'd also include bubble solution, because that's a fun activity they can do after, and also cornstarch in a zip-lock bag. The kids add water a little bit at a time, until they get a slime that is dry and hard if you close your fist around it, yet runs through your fingers when you let go. This is very messy and ideally you'd only do it outside. If you're doing this make sure it is cornstarch made of corn, not wheat, because wheat won't work.
The bowls hold a lot so you can get everything in.
Hope you can use some of these ideas!