You could just say that you're going out with friends and if she specifically asks where, you can always say it hasn't been decided yet. If you're going out alone (and no you don't need to tell her), you can give an "I don't know yet".
You're right, you don't have to be rude. There may come a time where you decide to give her less info on what you're up to. It's up to you how you address it.
I'm not a big player when it comes to mystery. I share sometimes, sometimes not, but it's about the moment.
