Firstly, it depends completely on where you go.
If you take a trip to London you will have to accommodate for the weather. It is has been very warm these past few days, and the last thing you want to do is wear a nice shirt and jumper and end up sweating profusely on the tube.
I would suggest a pair of brogue's, jeans (slim/skinny indigo) and a shirt would suit most occasions. A classic stone-coloured Harrington jacket would work very well over the top, and it is always easy to sling into a bag if you are too hot.
A pair of minimalist white leather trainers (not sure what your converse's are like), no socks; with a grey t-shirt or pattern shirt and light-washed jeans would also look pretty good.
Considering you are aiming to impress her, I would have thought you had already done that throughout the years you have known her and thus she really won't care about your wardrobe.