I would suggest (as I did before) that you say "Hey, I'm going out to dinner and to watch "XYZ" play on Friday, you're welcome to come along if you want." The whole idea of the picnic is you have a life independent of her, full of fun activities. She's welcome to join if she wants, if not you're going anyway.
as you know, i do not advise you ask her out... however, if you do, do not say it like this... i have been asked out like this in the past, and i do not like it... it's too vague... it's the, "if you want" part that is confusing... if you are going to ask her out, then make it clear... stronger...
