Charlie was a professional at entertaining himself. He excelled unbelievably at occupying his mind when there was nothing else around. He liked to sing (poorly), dance (worse than he sings), and sometimes he'll venture into sports or annoying his family. Swings were a less combative way to pass the time while he waited for Oliver to get back.
That, and he was amazed he fit in the swing.
He heard his friend approach and waved with one hand as he continued to swing back and forth a few more times before jumping off and landing a foot or so away.
"You know me," he replied smiling.
Charlie put his hands in his pocket and looked around. This had to look suspicious at best. Good thing no one was driving around looking for trouble to bust. This had drug deal written all over it.