I've done enough hiking with young ones, now, to know never to leave home without some sort of bag to keep our found "treasures" contained. Tired of carrying things in my own pockets or tucked away in the stroller somewhere, I made mesh type hiking bags before our trip to the Smokies last spring (reference here).
Once the nature bags are emptied...preferably outside...a sorting of sorts can begin. Paul has begun talking with Jack all about categorization in the sciences and this is a fun and easy way to aid this discussion. Guided by the appropriate age levels, I've helped the kids narrow down everything in their nature bags to categories. Then they choose one category to glue on to an old ice cream lid (with paper inserted so it is reusable) and research what the items are. Certainly there is alot of "hand holding" in this process the first few weeks. We're slowly assembling reference materials and such (besides Google) so further research will be more child directed.
Lena's Wildflower Collection
Jackson's Leaf Collection