I don't think the opener needs to be too clever / funny. If it just comes out, off the cuff, then fair enough but otherwise I would keep it as simple as possible. All your doing with the opener is giving them the signal that their about to be engaged in conversation by a normal person, albeit a stranger. I think as you get good the playful side will come out as you relax more.

I think because it's so unexpected in the day she's probably forgotten what you said in opening within a few seconds anyway. It's how quickly you can settle into the convo that makes the difference.

I found I was getting more nervous as the convo progressed instead of more relaxed, then bailing, like you are doing. This is a result of going inside your head and not being fully present in the interaction. But I think that was just down to me not doing it enough. I'm trying to strike up convos with strangers when I'm going about my normal day until it becomes more normal to me to do this.