don't respond... which might piss her off... and there's other negatives like, you'll maybe not get to know what's on her mind...
or respond with a simple, "sure, when"...
or you could say... "I've got a few things that I have going on this weekend... when would work for you... what's on your mind...?"
Then you get to let her know you have a life and it's good and busy (don't lie, make that happen) and also it opens up the possibility that she'll tell you why she wants to chat... and then if it's bad... you can choose not to meet...
