Neither of these suggestions seem to exactly fit the level the OP was aiming for, but I add them for others who come across this thread with a different group of students in mind:

(1) For a gentle, problem-based introduction for undergraduates, I really like Ken Bogart's [Combinatorics Through Guided Discovery][1].  Sadly, he passed away while writing the text, but he has left it publically available at no charge.

(2) For a comprehensive and structured approach to combinatorics at the introductory graduate level, I really like Martin Aigner's [A Course in Enumeration][2].


  [1]: http://www.math.dartmouth.edu/news-resources/electronic/kpbogart/
  [2]: http://www.amazon.com/Course-Enumeration-Graduate-Texts-Mathematics/dp/3540390324