A good job for mindless physical activity: cart-pusher at Walmart. I did this in high school, and it is still easily the best job I've ever had. You work at your own pace, you're outdoors, the managers usually ignore you (so you don't even have to obey dress code). Mostly I just screwed around with my coworkers.

Basically you just spend all day walking (with occasional bursts of hard physical exercise). I lost 40 pounds in the first 6 months and got into the best shape I have ever been in. There's also a kind of pleasant exhaustion after putting in 8.5 hours.

ETA: These jobs are extremely easy to get: even though I lived in an economically depressed area, I was hired without even an interview. A woman from HR called 90 minutes after I submitted my application.