Oh, congratulations on your first cruise!! You will love it! We've cruised Royal Caribbean numerous times and really liked it. A nice dress for formal nights is fine. You'll see the whole gamut, tuxes, suits, dresses, pant suits. One one of the cruises we took, we decided we didn't want to do formal nights so we just went to the buffet those nights. Comfortable shoes are the most important thing - you'll be walking a lot on the ship, as well as on shore. Shorts/pants and t-shirts during the day.
The advantage of taking a tour sponsored by the cruise line, if you are late getting back for any reason, the ship will wait for you. If you go with an independent cruise company and your bus breaks down or something, the ship will leave without you. Look through all your options and decide what you are most interested in. If you are into snorkeling, etc. the packages are pretty much the same for each island. Or you could take an island tour, just to get the flavor of the places you are visiting. Most of the cruise ships dock close to the town, so you can go walk around on your own.
The cruise ships are all "cashless" now, meaning everything gets charged to your room and you get a surprisingly ginormous bill at the end of the cruise! Two years ago, when we traveled Royal Caribbean, was the first time we had to pay for soft drinks, that was surprising. You used to only have to pay for alcohol, but they want to get money from you whenever they can. If they offer a drinks package, sit down and figure out how many drinks you might drink in a day and compare it to the price of the package. Usually, if you buy the package ahead of time, you don't have to pay the 15% gratuity. They automatically add that to each drink when you get them separately on board. Sometimes the cruise line lets you bring your own bottled water on board. We've done that and it saves us a lot of money.
Massages - they are fabulous on some cruises, so-so on others. It just depends upon how good the masseuse is.
Keep some money aside to pay tips at the end of the cruise - wait staff, dining room manager, room stewards, it seems everyone has their hands out. That NORMALLY ensures you get treated well. We are taking a cruise in November on a line that doesn't allow tipping. Should be interesting to see how we are treated on that ship.
Things to do on board. Aside from the regular stuff like swimming, sunbathing, shopping on board, they usually have a lot of stuff to do. Bingo, cooking demonstrations, island information meetings. But my favorite? TRIVIA! They usually have two or three trivia games a day, sometimes they are themed. Play by yourselves, join groups - whatever your heart desires. We have met some of the nicest people that we played trivia with all week. Let me tell you, people take their trivia playing seriously! All for a Royal Caribbean key chain! you can do as much, or as little, as you want on board.
Will there be a costume party or something for Halloween? If you can fit something in your suitcase, great. Play on words are easy costumes: Write BOOK on your forehead (Facebook), use face paint to paint a black eye and color a large letter P on your shirt (black-eyed pea), wear a slip and put a nametag on that says Freud (Freudian slip), glue cotton all over yourself and carry a small spray bottle of water (cloudy with a chance of showers. Squirt people when they ask what you are), print a picture of a past or the present Queen, put it on a piece of cardboard to make it last longer, attach a string to it and drag it behind you (drag queen), wear scrubs and borrow a pepper mill on the ship and use a string to tie it around your neck (Dr. Pepper), put a cereal box around your neck with ribbon. Stick a fake knife through it (serial killer). Wear a hat with a leaf hanging on it in front of your face. Blow on it when people ask who you are (leaf blower). Attach random clothing articles all over you (static cling). Buy a pig nose at a costume shop - borrow a sheet on board and wear it like a toga (pig latin - speak igpay atinlay all night). Just a few thoughts.
Packing for the trip - I've found that I never wear everything that I take. Sometimes you'll put on something just to wear to dinner. You can certainly wear that again another evening. I pack and then take out 1/3 of what I've packed. Make a copy of your passports just in case they get lost. You usually have a safe in the room that you can keep them in, along with any other valuables. You can usually empty your suitcases and hang things up/put them in drawers and store the suitcases under the bed and out of the way.
Things to bring - an over-the-door shoe hanger has been our lifesaver. The rooms are so small and there is never any storage in the bathroom. I put all our medications, suntan lotions, combs, razors, etc. in the hanger over the bathroom drawer. Bring a power strip. There are never enough plugs for things in the rooms, especially if you want to keep Kindles, Nooks, iPads, iphones, etc. charged. A small sewing kit (just in case). Bonine in case you are afraid of getting seasick (better than Dramamine because it doesn't make you tired). Febreeze. If you are a big coffee drinker - bring your own giant bubba cup to fill up and carry around with you.
Just a suggestion - Loooonnnnnggggg hallways of doors. I always go to the dollar store before a trip, pick up some magnets and some decorations to decorate our cabin door. Or you can make a clever sign and put it on the door. It makes it easier to find your room.
That's all I can think of right now. Sorry it's so long-winded, but mention 'cruise' to me and I'm off! Have a safe and wonderful trip!