Yes, the teens seem to enjoy the dressing up too, so pack your nice "formal" dress. I've seen lots of prom type dresses on formal nights, I just prefer a shorter cocktail dress with sparklies, just wear what you are comfortable in.
Jen, check out these packing lists. They are funny too. You certainly don't need to bring everything. Nice shorts or capri's with tops for daytime, 2 formal nights, Smart Casual outfits would be skirts, sundresses, mini shirts, nice pants outfits, and Informal if on your cruise is a nice dress or pants suit something that you might wear to a daytime wedding. Don't forget 2 swim suits with a coverup for those cool halls plus comfortable shoes, sandals and sneakers. Jeans are really too hot for the Caribbean so leave those at home. Shorts are not appropriate for the dining room at night but are OK during the day
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!!!
Jen now is the time to hit the major department stores special occasion or dressy dress section to pick up holiday outfits really cheap. A simple long sheath dress with a bit of sparkle would be perfect, steer away from really frilly, frou-frou type ball gowns, you can't go wrong with a perfect little black dress, hose & heels