I smile back, :-) and if I am single, maybe wink ;-)
If the time is right or appropriate, I would with follow generic small talk...
"Beautiful day isn't it? My name's Matthew Lee..."
I've never been great conversation in situations like this involving beautiful women and meeting them unexpectedly for the first time, I am, by years of proof, the definition of the shy, silent kid in class. I still try every time though, hoping my speech impediment from 3rd grade doesn't greet her instead.
"Where are you headed to right now? Any chance I can follow you? No, you don't need to use your mace"
Hahaha, just kidding. I love talking with women, and being open, honest, and talking to them like anyone else is the way I do it. There is no reason to treat a beautiful person of your desired sex any better than their opposite. Although we might all do this anyways, I make an effort to seem like my interest goes only so far, even if I am lying.
Being friendly and social is important and fun. I wish I had the mentality I use now, to speak with girls when I was in high school. Wow back then it was a challenge for me!