LoL @ the other responses. However, I'll be slightly nicer in my response since I used to ask the same questions (and trust, when you get good at this you'll look back and wonder how you were able to ask such AFC questions w/o hitting yourself in the head).
You may have to start out by "faking the funk," i.e. making up stuff. However, the best way to not have to lie about having stuff to do... is FINDING STUFF TO DO.
Seriously - go on to Yelp.com and find out about events happening in your city. It's a great site to use when trying to find interesting things to get into. Then, just start doing those things. At the very least, always have one or two things you'd legitimately like to do on the weekends, regardless of if someone is going with you or not.
Then, when the girl asks you, "so, what are you doing this weekend" or "do you have any plans," you'll have something to tell her - something you're actually DOING.
Right now, though, if she asks you these questions she's not doing it to see if she can tag along - she's doing it to see if you actually DO stuff, as opposed to sitting at home all day. A girl wants to know that any potential guy she's thinking about doing things with is doing more with his time than going to work and being a home body.
You'll know when she's ready to tag along when, after telling her about what you're getting into, she tries to invite her self along - something along the lines of "Ooh, I wanna go!" or "Are you going with anybody?"
Again, I'd like to stress that with girls you work with it's a bit trickier. Therefore, it's better to let HER determine the pace at which she's ready to see you outside of work. Just make sure to listen for key phrases and signals, and you'll know when it's time to make that move.