A Cruise is probably one of the coolest vacations you can take. To address your concern about off ship activities. Ask yourself, are you the adventurous type who is willing to walk off the beaten path to find the "local" sights or do you like to be more structured?
Don't sign up for any of the sanctioned shore excursion they are overpriced and the guides just want to take to you shops owned by their brother.
If you want to do something fun, wait till you get off ship and there will be plenty of vendors trying to sell you services, Scuba, Sailing, Fishing, Hiking Ect.
Check them out, haggle a bit and have a great day. Just watch the time. Last thing you want to do is that mad dash to the ship as its pulling away from port.
Pick up some of those behind the ear motions sickness discs from your Dr before you go. You may not need them, but you will be super glad you have them if you do.
And one last piece of advice. Depending on the package you bought drinks may or may not be included. In most cases when you board a ship you hand over a credit card number and are issued a ship card to make purchases. Its very easy to loose track of how much you spend on those things, especially on drinks as they like to tack on a tip and a service charge to each and every drink, order several at a time to avoid a million small charges.
Have fun, and consider me jealous.
The biggest purchase I made recently is the cruise of the mediterranean I am going on in February. I've never been on a cruise ship and I can't decide if I will actually like it or not. I don't like overtly touristy stuff; I prefer to see the real local areas and do hikes when I travel versus going into gift shops and sites setup specifically for tourist. I think the engineer in me will really like the boat but the tourist inside will not. I couldn't pass on this opportunity because a bunch of my friends are all going; it will basically be a week long party together.
Picture is of the ship we will be on.