I've been on a couple cruises and found them to be some of the most relaxing vacations I've ever taken...mostly because there's only so many places you can go on a boat, so you're kinda forced to take it easy.
I always enjoyed the anytime seating at dinners where you get to meet different people and find out where they are from. Checking out the different seminars and performances are fun. Typically have good acts - and there's usually a lot of diverse entertainment, from classic piano bars to upbeat nite-clubs. Treat yourself to a massage if you can afford it, or check out some of the off-ship excursions.
If you are planning to do any shopping, it's worth going to some of the shopping seminars as they can give you discount vouchers at businesses partnered with the cruise line.
The deck can get very windy, so be prepared for that. Most ships have wind breakers to keep most of the breeze off, but can get pretty chilly if you are walking the deck at night.
There are always reports of people getting sick, but I never had a problem. Just do common sense things - wash your hands often, drink plenty of water - and you'll be fine. I always liked the 24 hour buffet lines as I always got a variety of fruit/veggies along with meats and other cuisine you don't typically get at home.
On the cruises I took, the first day people were selling special "drink" cars - where you could get all-you-can-drink soda with a special container you get, and they had a coffee punch card. As I am a coffee drinker, I typically take the coffee card as it does save you money if you use them up vs paying for each drink throughout the cruise.
Most cruises now use an electronic card - for your room, for checking in/out of the ship on docking days, for charging extras to your room - you might want to have some kind of lanyard to wear to put your card in to keep it handy.
When you first get on the boat, you'll probably feel the rocking to some degree. I typically got used to it really quickly and only "felt" the waves on a really windy day (just one day that I can recall). If you do find yourself getting a little uneasy, don't hesitate to see the ships doctor for some dramamine. If you don't feel you need that, simply stay near the center of the ship where it rocks the least until you feel better.
Last tip I learned that I always pass on to people - if you are a jewelry lover - particularly those with gemstones, it's better to buy the gems unset (not as a necklace or ring or earring). The reason for that is unset gems are considered "stones" by customs and you won't have to pay customs tax on them when you return to the country; however it they are set - jewelry is taxed at customs. You are allotted a certain amount before you pay the tax, but this is a tip in case you are looking for investment purchases. Just buy the stones, then get them set when you are back home.
Hope you enjoy your trip!