10-15 push ups: it's because pull-ups are taxing for an 8 year old, using my brother for an example who can hardly lift himself up. Sit-ups are taxing on the back due to the amount of muscular endurance and strength while a mile running-I laugh- converted to meters is approximately 1609 meters. I'm in Junior high yet I can only do 900. Thus, it can be concluded the most appropriate activity a third grader could handle is a simple push-up.