Prepare lesson plans which take advantage of the fact you have one or more graduate students in the classroom. Reduce the amount of time you spend lecturing and use more student-centered activities in your class. Try placing students in small groups and have them brainstorm answers to question that arise in the subject matter you are teaching.
Create assignments that address the varied levels of ability in your class.For instance, when assigning essays, create a range of writing prompts with varying degrees of complexity.
Make sure to make the graduate student(s) in your class feel comfortable with their classmates and try not to single them out. Remember, even when you are exclusively teaching undergraduates, you still have to effectively instruct a range of abilities; this situation is not really all that different. Enjoy reading the sophisticated essays and thesis your graduate students are likely to produce.