It was long time ago, but I remeber [Combinatorial Problems and Exercises][1] by Lovasz fondly. However, this might be too challenging for undergraduates.   


  [1]: http://www.amazon.co.uk/Combinatorial-Problems-Exercises-Edition-Publishing/dp/0821842625/ref=sr_1_1?s=books&ie=UTF8&qid=1337236733&sr=1-1