You probably have heard about all three, Wikibooks, XQuery, and Sudoku, but maybe not in the same context.
Wikibooks is a wiki-based collection of free educational text books. One of the books is targeted at providing XQuery examples. Even if the system used for most of the examples is not (yet) DB2, it is worth a look. How come Sudoku puzzles into the big picture? Well, one of the chapters in the XQuery book deals with "Puzzles, Maths and Games" and one example shown is how to solve Sudoku puzzles using XQuery.
As I said, the examples are not based on DB2, but it should be possible to port the functions and query to DB2 (ping me for ideas). If you are teaching, writing something similar (from scratch) could be a nice student project.