I went on a massive photo scavenger hunt in the city once, organized by my scholarship program. Unfortunately, to level the playing field, each team had the same crappy disposable camera, so our creativity did not get the "exposure" it deserved and we kind of lost "focus."
Don't have all the items be regular things like fire hydrants or buildings, have open-ended clues or sayings that can be interpreted. For our hunt, one of the clues was "How did you all fit in there?" which is a bit obvious, so we all piled into this taxi cab booth thing. The lighting and picture sucked, so you couldn't see us in there at all, but you did get to see the police officer walking over to kick us out and yell at us. Other sayings were like "Only in New York," "Tasty," "Naptime," or "Circles!"
Also, it may be best if we get a group of unbiased judges who aren't participating to rank the pictures and actually have a winner. That just makes it a bit more interesting.