The kids spend hours bouncing, passing and creating new games. A large board secured to a fence or wall can act as a backboard for practicing soccer or tennis skills. If it's under shelter, paint it with chalkboard paint and the kids can create for hours. Add a steering wheel to a tree or fence post. Remind kids never to tie ropes to climbing structures and check for possible entrapment or strangulation dangers.
Sand boxes provide great opportunities for creative, cooperative play and fine motor development.
Be sure to keep your sandbox covered when not in use to keep the sand clean and cat-free. Consider moving some weatherproof indoor toys outside for periods of time. My kids actually started to use their play-kitchen once we moved it outside. Granted I'm still cleaning sand out of the crevices, but well worth the hours of fun they enjoyed. A most important addition to any backyard playground is a comfortable place to sit.