I never know what to say, either. From what I've noticed, people always say "Good, good", or "Fine, you?" or they make a silly joke. I usually say something like, "tired", "cold", "hot", "sick", or if I'm actually doing well, I'll say "good."
Usually what I say comes out sounding awkward. I can guess by others' reactions when I've said something that seemed odd to them. I'm really good at reading emotions and facial expressions, so I always know. That means that I often feel like a doofus. People will laugh, somewhat uncomfortably, at something I've said even though it wasn't meant to be funny. Or there's a pause before they say something, and they seem taken aback somewhat. I get all the cues that I am not doing what's expected, but can't figure out how to do what's expected.
Mentioning the weather is always good though, as you said. People love to talk about the weather. It's like the NT secret code that let's them know who's part of the group. So I can say "Oh, I'm a little bit hot, the sun is really shining today," or "I'm still freezing from being outside, it's been so cold all day."
I said something completely honest that totally threw off a cashier a couple of days ago. It wasn't a bad or inappropriate thing to say, but very blunt. She was obviously surprised but thought I was funny. I guess I was trying to be somewhat funny, but didn't realize that it was not something I should have said. So there is one more person that thinks I'm weird, haha.