Another variation we did on a treasure hunt was for my daughter's eighth birthday. I saw this idea in a magazine, but I've forgotten since then which one.
We bought spools of ribbon at a local craft store. We strung the ribbon over the entire back yard, one color for each different person. The string went up, over, down, around, through...til the whole yard was covered in masses of different ribbon colors. At the end of each piece of ribbon we had brown bags filled with treasures. We included books, pencils, stickers, and other small trinkets.
Each child was given the spool of his/her color of ribbon. They had to follow their ribbon's path, rolling it back up onto the spool as they went until they found their treasure bag.
We did this with ages 2-10 and each one loved it. We labeled each bag so we were sure each child got a bag with appropriate prizes for him/her.