What is the best exercise for baby 6 months old?

I don't think a 6 months baby requires actual exercise.... although I play a lot with my 6 month old son to get him to tire out. During tummy time, I'll place his favourite dinosaur toy just slightly out of reach of his hands and he'll do his best to get it. Sometimes we'll clear his mat and let him roll around happily until he gets tired and frustrated and wants to be carried. I also do stretch and massage him limbs after his baths. My late grandmother always says that it helps his legs to grow longer but I don't know how true it is. My son loves it though!