Basket and Disposable cups, paper and glue
Simply fill the basket with the cups. When your child is in the woods allow them to pick up flat objects or things with at least 1 flat surface. They can organize and stash their loot for later to examine. Plus as they find more things, they will get spurred on to find more. Great motivation to go more than a few blocks!
When you get home, you can take the objects from the cup and allow your child to put the objects on paper with dabs of glue. I recommend flat objects like leaves, bits of pine tree or grass and flowers.