We did this same itinerary last winter on the Carnival Fantasy out of Charleston, and although the Sensation is basically the same ship, Sensation provided far better service, and overall experience than the Fantasy.
We chose this cruise for 3 reasons: 1) Half Moon Cay and Grand Turk have great beaches to just lay back and relax, particularly Half Moon Cay which has the softest sand we've ever experienced (been to Nassau too many times so didn't get off the ship there); 2) the splash works water park on the ship keeps our 2 kids (6 & 8 yrs) entertained for hours; and 3) Camp Carnival has terrific programming for younger kids, so we adults can have some free time too!
We paid for the Faster to the Fun to get early embarkation and debarkation. It is well worth the cost! Very smooth arrival and departure. If you have kids or a group I highly recommend getting it.
Cabin R72 (deck 4) was fine. 4 of us in the cabin, 2 adults and 2 kids. Always cleaned well and promptly. Kids loved the pullman bunks, and the obligatory towel animals :)
Stateroom attendants were very cordial and attentive to any requests we made. They worked so hard! Thank you so much for making our cabin comfortable!!!
The beach is great, so we just rented a clamshell for shade and enjoyed swimming and a bit of snorkelling along the breakwater. Saw enough fish to keep the kids happy.
Rented an umbrella on the beach and enjoyed the water. Did some snorkelling with the kids and saw a few fish and crabs. Everyone was happy :)
Been to Nassau too many times. We stayed onboard and enjoyed the splash works water park.
Did nothing in Miami except transit through.