It is problematic when the young students put the books back in the wrong place. But sometimes I would leave the books to be shelved on the floor right in front of the shelf they belonged on. They were more likely to make it back to the right place, it gave them a chance to try and find the right place (with a little guidance), and if they do end up misplaced you know they won't end up too far away from where they belong.
They also love choosing what book to display on top of the shelves. Let them swap out books that have been on display for a while for something new.
I saw a neat library display (one that I haven't tried) that displayed books that had covers all the same color. A shelf of red books. A shelf of blue books. Young students could create that display just to be visually striking.
I also think 4th and 5th graders could be great at choosing recommended reading for the preschoolers and kindergartners.