A simple summer dress or a skirt and nice top. I think the pixie cuts tend to look more sophisticated and elegant (when girls make them work, which many don't), so I'd say stick to feminine styles. But then again, that's MY style.
Just a coservative top and jeans; you don't want to distract people's attention from the pixie cut, right? They should focus on that!
I have no idea what would be considered extremely dressed up for you but here are my suggestions: where lighter colors more, wear skirts sometimes, wear floral prints, maybe wear a ribbon in your hair sometimes, maybe some jewellery.