What worked for me was that I had each student take a book and 'establish' a
strong theme on the cover, and the endsheets. They then put the books on the
bookshelf. Then when kids are finished early with their "other" work, they
go over to the bookshelf, pick up one of the books other than their own, and
produce a piece of work on a double page spread based on the established
theme. By the end of the year, each student should have a book with their
classmates responses to their designated themes.