I have to admit the whole concept of your mission doesn't complete gel with me but since it's not like you're hiding anything from your girlfriend and she obviously approves since she's driving you to it I have no reason to really be against it -

Ok I have a feeling I will get replies detailing some in depth how to pick up girls guide saying you already know what your strategy is but just in case you haven't thought about it:

Have an excuse - like needing directions, where to buy something, asking the time etc etc since we're all pretty much told not to trust unknown guys who approach us and without some 'safe' alterior reason for making an initial approach you'll put girls on guard immediately.

I'm not really sure how big you are in person but if you look physically capable of over-powering people or have tatoos or anything which signals not being incapable of handling yourself in a rumble this is especially true. Sure it can be a selling point but if you don't make it non- threatening to start with it'll work against you.