My kids are obsessed with America Ninja Warrior. Even the 3 year old will hang from the monkey bars and yell, "I'm a Ninja Warrior!" at the top of his lungs. The 8 and 6 year old seek out playgrounds that might just be suitable as Ninja Warrior obstacle courses.
We stopped in for a kids class and for $10 each, the kids had 2 hours with the instructors. They did obstacle courses, with many of the challenges you see on the Ninja Warrior course.
I sat at the back bench, in the middle of the action, and watched the kids try all the obstacles. James wasn't old enough (5 years old) to participate in the class, but he was able to do a few activities while we watch the big kids. (His only concern was that there was no water or buzzer when kids fell.)
And look below for an introduction to the instructors you will meet at the gym!