Content modifications can include adding (or substituting) abstractions, complexity, variety, organization, study of people, methods of inquiry, and a variety of forms of subject-based acceleration.
Notice the emphasis is not on quantity, but on variety and depth. Gifted students can take an objective to a new level. Instead of tutoring the struggling student, they should be given an exciting and challenging alternative. Here's a challenge. Pick a large assignment that you are going to give before the year is out and make a concerted effort to make sure that your gifted students have a choice from one or more of the options above. The results will be exhilarating for the students and teacher alike.