Walking on a glacier in Alaska. Swimming with dolphins in the Caribbean. Joining a lobster bake in Maine. Pondering Michelangelo's statue of David in Florence.

Shore excursions during a cruise vacation offer a slew of opportunities to experience the culture of a destination, mix with local people, view wildlife and have some fun outside the confines of your ship.

Beware the costs, however, since booking an excursion at each port of call can add hundreds of dollars per person to the total cost of a vacation. Shore excursions generate lots of revenue for cruise lines, but there are options to spending more than you bargained for on shoreside activities. Here's how...

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Comparison shop
Cruise lines typically charge roughly 20% more for shore excursions they organize, compared to third-party vendors such as Viator.com and ShoreTrips.com. A half-day, guided walking tour in Venice, for example, costs $100 per person at Viator. A virtually identical one offered by a cruise line is priced at $129. A whale-watch boat trip in Juneau, Alaska, is priced at $135 at ShoreTrips while a similar cruise line-sponsored outing costs $159. In some cases, however, a cruise line can offer a bargain. We found a four-hour tour of Aix en Provence, from the port of Marseille, priced at $93 at Viator, but one cruise line offers a full-day excursion for $79, so it is important to do some research before deciding.

Use mass transit
In major European cities, buy a hop-on-hop-off bus pass. It will cost a fraction of what a guided tour will cost, either through a third party or the cruise line. A public bus also enables you to stop where you like, for as long as you like, and get a feel for the local culture. Take along a map and a guidebook, and you're all set.

Find local guides
In most ports of call, local tour operators will wait dockside for customers. You'll see them holding signs advertising their tours and prices. Rates will usually beat the cruise line's similar tour, but before hopping into someone's tour van, settle the per-person cost verbally, agree on the return time, and make sure the driver will bring you all the way back to the dock.

Take a taxi
Local taxis are a good option in the Caribbean. A cruise line typically will charge at least $20 per person for a beach shuttle while a taxi or passenger van to the same beach can cost as little as $5. Throughout the islands, taxi drivers are accustomed to transporting cruise passengers to a nearby beach and then returning to pick them up at an agreed upon time. Again, settle the fare before leaving the harbor, and always leave extra time for the return trip.

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Get advice
National, regional, or local tourist offices frequently have a visitor center or an information kiosk at the cruise port or nearby. Drop in and talk to the tourist representatives, who are always happy to suggest a tour with a reputable guide, provide walking maps, point out sightseeing options and recommend local restaurants that won't break the bank.

Drive yourself
Renting a car is a good option in some ports, but not all. For example, the cruise port for Rome, Civitavecchia, is a two-hour drive from the city. But in Nice, the cruise ship dock is just steps from the old town, and gorgeous countryside awaits just a few miles outside the city.  Save on the cost of a rental car by sharing it with traveling companions or with new friends you've met on your cruise. And remember that gasoline in Europe costs roughly three times what it does in the U.S. Factor your planned distance and gas costs into the total expense. Split between two couples, you'll likely still come out ahead of the cost for a guided motorcoach tour and you'll have an original adventure.

Be a VIP
A private car tour can be a good option for a small group, such as two couples or a family of four. But shop wisely. There are big differences in price between cruise lines and third party vendors. ShoreTrips, for example, sells a five-hour private tour of Cozumel for $109 per person. Some cruise lines offer seven-hour tours, with rates of $600 or more per vehicle. In some cases, the vehicle accommodates only three people, which brings the per person rate to a whopping $200, nearly twice the cost for an additional two hours.

Set your budget
Plan ahead and spend money only on your must-do excursions. Your ship might visit four, five, or six ports during a weeklong cruise, but that doesn't mean you have to buy an excursion for each one. Remember that it's always fun to knock around a port, try some local cuisine, check out souvenir shops and take loads of pictures while exploring a destination on your own. Don't be surprised if these become your fondest memories!