I love having a buddy class at school. When I taught in the primary grades, I recruited an older class for different activities. Now that I teach upper elementary, I usually adopt a younger class for my students to work with.
One of the activities I would suggest is reading the book Big Bad Bubble and creating a book about our childhood fears. I read the book to the older students and they create their own page. Then we share the story with our buddy class and my students help the younger students write their pages.
(This might be a good activity to introduce theme to both grade levels.)