We recently took our first cruise on an Oasis class ship(Allure). We had also read passengers posts where they said 7 days was not enough time to see everything. By day 3 my wife and I had explored and seen everything on the ship we wanted to see(we had 2 sea days in a row). So we weren't really sure what kept others from not seeing everything.
Perhaps they meant they couldn't see and do everything in 7 days. It would be hard to do all the activities and see all the shows. Most of the activities are on 5 or 6 floors out of 17 for the entire ship.
As for the ports. We like port heavy cruises. For us, a cruise is the opportunity to sail away to a different island at every port stop. So we always get off the ship and at least walk around, sightsee, shop, try a local beer or drink and usually eat a bite.
We'll never forget the time we stopped in St Kitts and they were having some kind picnic/celebration in the park that was just a couple of blocks from the cruise port. There was a parade, kids, families, locals selling grilled foods, fresh fruit drinks, crafts. It was just like what you would expect to see in any small American town. But it was all about the people of St Kitts, a Caribbean island. If we would have stayed on the ship we would have missed it.
If you haven't traveled a lot, I encourage you to get off the ship and have a look around for at least an hour or two. And maybe even pick up a sovereign so you can always have a memory of your visit. My wife has collected a refrigerator full of magnets from each of the ports we've visited over the years. I always try a local beer and I try to buy a bottle of hot sauce.
You can always stay on the ship if you want. But you're going to be on there for days anyway. Why not get off at port?