It's easy to rack up a large bill on a cruise if you're not careful. As another poster mentioned, be sure to account for tips. Carnival automatically adds tips onto your bill. If you want them to remove the charge and handle tipping on your own, you can go to the concierge desk to have it taken care of. You will constantly be offered drinks. As soon as you sit down somewhere - by the pool, in a lounge, at a show, on deck, a server will be by to ask if you want a drink. Even though food is included in your fare, there are little specialty coffee/pastry places around that sell tempting little goodies.
We took a Carnival cruise two years ago, and I recall that a soda card was $10 per person, per day. Sodas are generally $3 each, so you need to drink more than 3 to make it worthwhile. If you like soda with lunch and dinner, and you order more in lounges or shows, you're covered. One thing that I do recall - if you have a soda card and order a soda with dinner, good luck getting a refill. The drink server would come by for our order, bring the sodas, and disappear. If we asked the waiter for refills, he said, "I'll send the drink server over." Never happened once in four dinners. I guess they see that prepaid soda card and aren't interested in dealing with you.
As I recall, in Grand Cayman the ships dock near town and passengers are tendered in. The coral reef makes it impossible for them to dock right next to the pier. It's a good place to get off the ship and walk around. Just remember that if you want to have lunch on the ship, you'll need to take the tender back, and then back to town if you want to walk around more. Many times we end up paying for lunch on shore because the ship was too far away, or we wanted to try a local restaurant. In Cozumel, the ships dock right at the pier, but it's a little ways into town. There was a bus that went back and forth, for a small fee, or you could take a cab. We took a tour that dropped us in town. We had lunch, shopped, and then took a cab back to the ship. It was just too far and too hot to consider walking.
Other possible charges to consider - souvenirs on the ship or in port, incidentals like toiletries, OTC meds, etc. that you might need (they're sold on board at a premium), on-board activities (some have fees), gambling/bingo.