Whilst the dress code is 'country casual' I personally see nothing wrong in dressing up esp if it is a special occasion. DJ is probably a bit over the top though.
I would say that "country club casual" is a perfect description for the dress code. Men were mostly seen in long pants with collared shirts (polo or button down). Women were mostly in nice pants and tops. The only night that felt "dressy" was the first sea day, when we had the Captain's Cocktail Toast. If someone brought a sports jacket or skirt/dress, they wore it that night. That is not to say that you did not see sports jackets or skirts on other nights, but if you did, you just thought they had eaten in one of the specialty restaurants that night (which might or might not have been true). I definitely do not remember any suits, tuxedos or gowns, maybe a sparkly dress or two on that first sea day, but that was not the 'norm'.