My DD8 did a Gifted Learning Links (Northwestern's equivalent) last summer. (The one she picked was a grammar class.) It was time-intensive enough that I discouraged her from doing one during the school year, even though it took her fewer hours than the course guide suggested.

My DD has a very low tolerance for scheduling, and a high need for downtime, though. She swims twice a week, and that about maxes her out for after school activities.