Sept can be a mixed bag but mostly on the positive side as end Sept to early Oct is my favorite time of the year. I wear shorts during day time into November. Days can hit 80 but also mid 30s. I would bring shorts, jeans, kakais, t-shirts, sweaters, etc. In other words a little bit of everything you own. Sorry I can't be more specific.
SusCie I don't know where you're from, but just make sure you stay casual! Even really formal restaurants here in NH are nice/casual. No need for the high heels or glitzy outfits. I am from Boston and am just continually amazed at how casual even "dressy" places are here - it's a very laid back place, wardrobe wise (not politically that's for sure).
Oh, and I forgot to say the key here is layers - bring stuff you can easily layer on and off, I agree with the expert it's pretty unpredictable.
FloweredCow nailed it. Layers are the key. Remember that the weather differs greatly with elevation and proximity to the ocean. It can be much cooler in the mountains and coast than it is inland.