Easy things one can always find outside: sticks, rocks, leaves, a snail, roll polly, and many more. We try to keep it to about 3-5 items, but add numbers to each: lets's find 3 rolly pollies, 5 leaves, 1 long stick, 3 short sticks, etc., to help get a numbers lesson in, too.
Sometimes we fill a basket to take the found items home for crafts, or leave them behind for the thrill of the hunt. I also like to bring my iPhone to take pictures that we can review certain and unique items later.
A scavenger hunt helps them think and focus, get some exercise and fresh air, and is an excellent way to get outside and enjoy some imaginative play.
Extra tip: We also do a nightly indoor scavenger hunt to pick up toys in the living room or my daughter's room. She has a blast cleaning up as mom and dad call out the items to find. It's a great way to reduce the clean up time battle.