This is one of the reasons that, despite the immense cost, we're trying to stay in NYC as long as we can. Two small children in an apartment definitely has its tough moments. But we live by a giant park packed with playgrounds, when we go we frequently bump into friends from school, meet friends of friends, etc. Everyone can go get an ice cream, head home happy. We have dozens of museums an easy subway ride away. etc…
In terms of serendipity, I'd put a college town far ahead of a large city.