I can hardly do better then recommend the 2 books by Milkos Bona: A Walk Through Combinatorics and Introduction To Enumerative Combinatorics.

The first book is more comprehensive as well as classical,giving thorough discussions of counting arguements and the intuition behind them in addition to bijection arguements.It also contains a very good introduction to graph theory and some topics not normally found in introductory books,like lattices and partial orders.

The second book has considerable overlap with the first,but the emphasis is a lot more on modern counting methods.It is more formal and less intuitive then the first,but the discussion of several topics is better and there are better exercises. The chapter on generating functions is probably the best single chapter source in the current textbook literature on the subject.

Using either or both of these books will give your students a terrific course.There's also quite a bit of material available online for free: Richard Stanley's 2003 Art Of Counting course at the MIT OpenCourseWare website has 233 substantial combinatorics problems for your students to chew on.

You also might want to look at the terrific classic by Wilf on generating functions,generatingfunctionology-available free for download at Wilf's website.

Anywho,those are some of my favorite books on combinatorics.