I've been on 7 and I LOVED THEM. I always went with a sig other and had wonderful memories. I always had plenty to do...I could lay out by the pool, play the many pool games that they had set up, play basketball, shoot skeet, take dance lessons, attend a live auction, go to a wine or chocolate tasting, go to the library and read a book, go the video game arcade or the gym. That's just onboard and now there are ships with golf, rock climbing walls, ice skating rinks and surfing simulators!
Most of the time I didn't even need to get off the ship when it pulled into port, since I've been to the Caribbean so many times. However, if you've never been to the ports of call, you really should pay for a cabbie to take you wherever you wanna go or take one of the tours offered.
Since you pay for all your food and drinks (no alcohol though--that's where they get you) up front, it really is more economical. Just watch the tour costs at the ports--they can be a bit of a rip-off sometimes. Bring your own alcohol (sneak it onboard) because your liquor bill could easily be $500-$1000 at the end of the cruise!
There really are a lot of things to see and do, but it's really up to you. You can be as busy as you want to be or be a big ole slug...it's totally up to you.