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. 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.