The convenience and benefits of having other children for my kids to play with are immeasurable.  Children think of great imaginative games to play (e.g. tricycle, scooter and bicycle races).  Children subconsciously value other children over things.  Yes, they fight over toys and things.  At the end of the day, they enjoy companionship more than any gadget or gizmos you can ever offer them.  Secretly, I've always looked forward to playdates as much as (if not more than) my children do.

So organize an impromptu play date or informal get together at a local park.  Bring a ball, bike and/or kite.  Just let them play.