Two obvious answers are van Lint & Wilson "A Course in Combinatorics" and Peter Cameron "Combinatorics". Which is best really depends on the fine details of your course, and what content you want. Cameron's book has a lot of nice exercises, there are not as many in van Lint & Wilson (and they have a tendency to go of the deep end). As you would expect both books are very well written and have an excellent selection of topics. Cameron's book is possibly more approachable.

Grahm, Knuth and Patashnik is a fine book, but is much more focussed on classical combinatorial sequences and less on combinatorics in general.