IMO food is the best and easiest reward partly because of the extinction burst you get when you fade it. If food works, I'd use it.
In my experience with other dogs in our training classes Charlie is a mix of two breeds for whom food is often not as good a reward as play. Obviously she likes the bubbles.
So cute. They could be a reward once in a while. I see many using tug as a reward, does Charlie like tug?
What does Charlie really, really, really like to do? Can you use that as your reward and make her work for it? It's not always an easy thing to do. A Siberian in one of our beginner classes was not impressed with food or play. But he loved, loved, loved belly rubs. That was his reward. Man, it was slow. Every time that dog did something good the poor owner had to give him a belly rub. That doesn't sound so bad but he had to be lying on the ground on his back. While the rest of us were all quickly advancing, using our food treats and one other dog playing tug, the Sib's owner was working on getting him to respond to belly rubs given while he stood.
And therein lies another suggestion. Are you taking Charlie to obedience classes? They will help. The trainer should be able to help you discover what Charlie will work for. The other benefit is that you introduce new people and dogs in a controlled situation that is usually easier to start with than out in a park.
Did I say I love the bubbles?