Our five year old son loves to run around our kitchen loop so much that one night he ran around it over 100 times (we counted). Not really ideal in a NYC apartment.
We're looking to meet up with other 3-6 year olds who love to run! Open to ideas on location - and we can move it around - but I'm thinking about the loop around the Great Lawn in Central Park for starters (or on the field once it opens for the season).
This would be super informal. Kids need their own supervision, water, sneakers, snacks, etc. We meet up on weekends or after school, and we run! I see this as a way for the kids to meet up and run/play together as well as for parents to engage with each other and with the kids.