The kids I nanny during the summer love scavenger hunts. They love to both set them up and to actually participate in them.
When I set up a scavenger hunt for them I usually make a trail of hidden notes. The notes usually say something like, “Look inside the mailbox” or “Check under the bed.” Each clues leads to another until the last one leads them to a little prize at the end. In the past I have used popsicles or small change as the treasure at the end of the hunt.
The kids also like to switch roles. They set up a scavenger hunt with clues that I am to follow. Either way they always have a good time with this activity!