I only let my dog off lead in the park when there are no other dogs in sight. As soon as another dog is seen she goes back on the lead. She is quite happy on the lead, but I like her to chase a Frisby to give her a good fast run.
I take her to our Country Park which she loves and then she is on an extender. She likes to sniff and never goes to the full extent of the lead. However it is quite thin and , having read another thread, not sure how safe it is. She wears a harness so the extender is attached the loop between her shoulders. She weighs 15 kilo if that helps.