Things for which you'll pay extra include alcoholic drinks, soft drinks (Coca Cola, etc), bottled water, specialty coffees such a cappuccino, shore tours, and specialty restaurant surcharges. Since all of the above are voluntary, you can regulate your extra charges by keeping track of how much you're spending on drinks, etc.
The drink packages are available but not mandatory. You can pay on a per-drink basis if you prefer.
Don't forget that you're going to have a fantastic time!