I think it might depend on what cruise line you're going on. I'm not sure what it's like on triumph, but we went on the Disney Magic and while some people were in ball gowns and tuxes for formal night, many were not. I didn't want to pack a formal gown for two reasons, the first being that the only formal gowns I own are the ones from Homecomings/Proms and they don't fit any more (not even the one from my senior prom in 2002, because I gained too much weight,
). I really didn't want to go out and buy a formal dress. I also did not want to deal with packing one. I had a hard enough time trying to fit all the other stuff I needed into my suitcase!
What I ended up doing for formal night was wearing this really nice black skirt, a red sparkley top and a black overshirt because they were easy to pack. Whether you chose to pack a fancy dress or something more simple, you'll probably see a wide range of styles on formal night. As far as other things to pack, just pack comfortable summer clothes for lounging around in, swimsuits, and lots of sunscreen!!!