Nature
Create a list of objects in nature for kids to find and photograph for their scavenger hunts. You will not want to send kids off alone in nature, so create teams with older chaperones who can take the kids around parks or walking trails to find certain types of flowers, specific trees and various woodland critters. You can even make the list specific, such as "Red leaf with a hole in it," and "Squirrel in a pine tree."
Food
Food can be a very fun and delicious theme for kids to search for in their photo scavenger hunts. Make this project last a few days so that kids can take time to find, and try, all the foods that are on the list. Combine a little nutrition into the theme, and create a scavenger hunt where kids have to find food from healthy food groups, including fruits, vegetables and whole grains, as well as not-so-healthy junk foods. They can be creative with the foods they find, and then show the other kids what food they took photos of and explain why the foods are healthy or unhealthy.
Neighborhood
It's good for kids to explore their neighborhood through the view of a camera lens. Have kids get in small groups and walk around safe areas of a neighborhood during the day to try and find unique characteristics they may not have noticed before. Kids can work off a list that has items like "A rose garden" or "people sitting outside a cafe" to get them to see where they live in ways they never have before.
Letters and Words
Signs are everywhere, so rather than treating advertisements and street signs as modes of distraction and direction, use them to help kids build their vocabulary and stimulate their creativity. Create a photo scavenger hunt where kids have to take pictures of letters or words from a compilation of street signs, store fronts and advertisements, to combine them into a string of words that tell a story. Give them a theme for the hunt, such as "Ways to Have Fun" or "The Epic Tale of a New York Taxi Cab." The more creative the theme, the more room kids have for personal interpretation.