First - Budget about $175 for tipping the 4 tipped positions - check the gratuity guide that
DCL has on their web site - search for "Gratuity" NOT "Tip"
Second - Stay away from Shutters

It's been our downfall too. If you do get pictures from then remember that they are 6x8 and NOT 5x7. Once you get home, frames are a problem.
Third - Drinks - they can add up fast. If you mostly stick to the drink of the day you will pay less (very resonably prices)
Fourth - Shore excursions add to the price very quickly. Don't know what you like to do but the Dig at the Atlantis hotel is very nice (and you can do it on your own for much less $$$ than through DCL)
Fifth - The Spa - you can spend a lot of money here but on a 4 nighter there is limited time. Not everyone does the spa (we haven't and don't think that we've missed anything)
Sixth - Palo - Some people say its a must do. We've not been to dinner there, just brunch (and brunch is GREAT!). There is a $10 per person charge for going to Palo. You must plan on additional $$$ for a tip.
Seventh - If you like wine with dinner, consider purchasing a wine package the first day on board.
Eighth - Turn OFF charging Privileges for the children (sometime they like to "buy" friends by buying others smoothies and arcade cards) Or set ground rules for them to follow. If you have an infant or toddler turn their card off too - heard here that someones DD spent a lot on her card and the parents turned it off, she then took her younger siblings card and racked up the charges again - wow!
Ninth - Souvenirs - you can spend a lot on all sorts of DCL branded items.
Bottom line - you really only need to spend money on gratuities. Everything else is optional and you really won't be missing out on anything if you don't spend a dime on anything else. All of these things can help you enjoy your cruise/vacation more but ..You can get off the ship in ports and wander around for free. You don't really need the adult drinks and the smoothies, you can take pretty good pictures on your own, etc.
The food from all restaurants is free (except for Palo). Room service is mostly free - items have a charge. Tons of entertainment on board - live broadway shows, movies, game shows and many more activities. There are the swimming pools and sports court for other activities.
Go and have a great trip!
Oh yea, someone else is doing the cooking, cleaning and picking up your room for you and you will be in the tropics

It doesn't get much better than that!