What's a good workout for a 13 year old
Go running 2-4 miles a day, play outside with friends and siblings. If you want abs do 500-1000 sit-ups a day. Join sports. Tae kwon do is a great workout, plus you learn self-defense.
running,skipping,playing games that moves your muscles(badminton,football)
Running, push ups, crunches, sit ups, stretches
Anything that doesn't require weight lifting you are to young to lift