Hi J.,
I planned a scavenger hunt for the kids on our block at one of our block parties and it turned out to be a blast. The children were split up in 2 groups with about 8 in each group with varying age levels. (You can split into as many groups as you want.) I made both of the lists the same so that it would be fair. I love the picture ideas you already received, that sounds like a hoot. I don't think I was that clever. The winning team got to pick some grab bag gifts I bought at the local salvation army. I think that part was the most fun.
Another year, I organized a watermelon hunt. There were 2 teams again with different clues to finally end them up at a waiting watermelon. The clues were like "go to the stop sign at the SW corner and read on to find the next clue." Of course, the night before I was out taping clues to stop sings and neighbor's porches and potted plants. The watermelon hunt definitely required more thought and time. For the teens, you can make it even harder and go off the block, but you don't need to. This year I am going to do bowling in the street. I collected 24 pins from bowling alleys for free and purchased a ball at a garage sale and we will have bowling for the teens and adults. There's a book called "Block Parties and Poker Nights" that you may get some other ideas, but I just looked and no scavenger ideas. Darn. Good Luck!!