I always kept Molly on a 30 foot light leash when in the same situation you explained. We would go to a local school yard for some off leash training and play and I would always keep that leash on her because if she ever did get distracted by a squirrel or bird then I had 30 feet of leash dragging behind her to grab to get her back to me. I would lure her in with the leash and make sure I was much more fun and entertaining than anything else on the field and then praise, praise, praise when she made it to me.