3. Shore Excursions
Booking shore excursions through a cruise line is easy. But these tours, which run from $29 to $150 (and higher for extravagant offerings), are priced to make money for the cruise lines. Planning can pay off. It's nearly always cheaper, for instance, to book through an independent local operator, and to use taxis or other local transportation for trips to the beach.

4. Alternative Dining and Treats
You'll find plenty of selections in the ship's dining rooms and casual buffets. But the lines also offer alternative, gourmet venues — which cost an additional $10 to $75. Eliminate this as a hidden cost by budgeting accordingly for a special night out or planning to forego it altogether. And watch those extra charges for name-brand ice cream, specialty coffee drinks like espresso, and other treats.

5. The Spa
Spas on most ships aren't cheap. Those who pay $22 for a pedicure at home will be shocked by a $70 shipboard tab, and a 50-minute massage will run at least $119. Look for spa specials on port days. Beware, too, of extra charges for some fitness classes — $10 to $15, say, for Pilates or Spin.