The answer to this question depends on what kind of photographer you are.  If you prefer capturing the details of your surroundings, a telephoto zoom or even a macro lens may be a good choice for you.  If you are a “big picture” photographer who prefers to capture sweeping vistas encompassing all you see then a very wide angle lens may be best for you.  For me, if I could only take one lens on a Baltic cruise, it would be a Canon 24mm - 105 f/4 IS on my 5D or, on a small sensor crop camera like a T4i, a Canon 15mm - 85mm f/3.5-5.6 IS. 

A Baltic cruise is largely urban exploration via cruise ship.  Most itineraries are very port intensive with many ports along the way.  Rarely will you get the chance to escape the city you are visiting to find wide open spaces and thus rarely will you need a telephoto lens longer than 135mm.  In a city setting you will probably find a moderate wide angle to moderate telephoto zoom will serve you well.  These lenses aren't particularly heavy so you can schlep one around on even the longest shore excursion.  They are also small enough that taking it in and out of your day pack or wrestling on and off mass transport should be easy.  Finally, a small camera/ lens combination means that you should be carrying a smaller camera bag.  This means should should hear, as I did upon entering a museum in Zurich, the dreaded words “zu groß“, which meant I had to leave my huge camera bag with the nice matron at the front desk rather than be allowed to knock around the museum with it.

The modern Canon image stabilized lenses promise to increase stability by three stops.  That means that an f/4 lens could give you sharp images as if your were using an f/1.8 or f/2 lens.  An f/3.5 - 5.6 lens could behave as if it were an f/1.8 - 4 lens.  This means you should be able to get decent shots even in dimly lit churches and museums if they allow you to take photographs.

Please remember this “three stop advantage” assumes you are a steady shooter.  Never trust this rule of thumb - check it out for yourself.  Long before you leave on that cruise, make certain you have the lens you are going to travel with in hand and visit your own local churches and museums that allow you to take photos inside.  Experiment with various shutter settings to determine just “how low you can go”.  You may find that, depending on lighting conditions and whether you are using the wide or telephoto setting or bracing yourself against something solid, your results vary.  Knowing this ahead of time will save you experimenting while on tour somewhere and prevent disappointment later.

I also pack a second, single focal length (prime) lens like a 50mm f/1.8, 35mm f/2 or the new Canon 40mm f/2.8.  While not really much faster than either of the lenses I have recommended (once the image stabilization is taken into account), a fast lens like this is very small and easy to bring along. It also does a much better job of delivering a very narrow depth of field that will throw the background out of focus and let your main subject "pop".  This effect comes from a wide aperture, not from image stabilization, so an f/2.8 lens will be able to provide a narrower depth of field than an f/4 lens used at a similar distance and focal length.

To sum up then, take a moderate to wide angle zoom lens, augment it with a fast prime and test your ability to handhold your camera and lens combination before you go.

On a beautiful July day, Jan and I explored Langelinie Pier in Copenhagen where Celebrity Constellation had  been tied up overnight.  We had spent the previous day walking the city centre and in the evening we explored Tivoli Gardens.  Tivoli is a jewel of an amusement park  that opened to the public in 1843 and if you are there at dusk, you will think you have stepped back into the mid 19th Century.  The rides are wonderful, if not as breathtaking as some might like, the restaurants are top-notch and many, and the live entertainment in the park will have you lingering well into the night.  In spite of the temptation to stay late at Tivoli Gardens, we had gone home early.  A good night's rest meant we were now ready for a tour outside of Copenhagen and into the picture perfect countryside.

After our morning exploration of the pier we boarded our bus.  We headed towards North Zealand along Highway 152 and through an area known as the "Danish Riviera."  Denmark is a country of eye-watering beauty.  The Danes have decided that aesthetics trump mercantile interests and the countryside is devoid of any billboards or commercial signs - including those of estate agents.  We turned off on Gammel Strandvej, never having our eyes assaulted by greedy commercialism, and made a photo stop along the Baltic Sea near what once were small fishing cottages.  These thatch-roofed buildings used to be the homes of humble fisherman but are now the expensive residences of the well-heeled.

We continued north to Kronborg Castle, located near the town of Helsingor (Elsinore).  The fortress here has guarded the narrow strait where only 4km of water separate Denmark from Sweden.  While it was meant to guard the Danish frontier, the Swedes walked over in the winter of 1658 and commandeered the castle.  When control returned to the Danes, they thought it would be best to increase the strength of the fortifications so that no one else walking by might decide to stop and occupy the castle.  As the castle is also known as Elsinore, it has had a close association with Shakespeare's play Hamlet and a number of performances of the play have taken place in the castle.

 Fredensborg Palace, the summer home of the Danish royal family, was our next stop.  The building, also known as the "peace castle" is where the Swedes and Danes signed a treaty promising to stop swapping castles with each other.  A long, descending gravel avenue call the Slotlet serves as the approach to this magnificent castle and seems to be a wonderful place to park a great many tour buses during the summer. 

This stop was relatively short and filled with tension as our tour guide herded us around the grounds so that we could arrive at our lunch stop before any of the other tour buses beat us to the buffet.  With the perfect timing that usually comes from any tour the Dougall's are on, our tour bus arrived dead last at the watering hole.  Regardless, as impressive as the morning's castles were, this restaurant was able to feed six tour bus loads of guests AND had sufficient cold beer (and washrooms) to accommodate every one in short order.

Our afternoon was taken up with a single stop at Frederiksborg Castle, which is also the Danish Museum of National History and the largest Renaissance castle in all of Scandinavia.  This castle looks and feels like the home of royalty.  From the huge fountain at the entrance to the massive chapel that survived a devastating fire in 1859, we could have spent the entire day there - and having walked most of the building, it seemed like we were there a long time.

By this point, we had done almost all the walking we needed to do on this particular tour and took a well deserved break on the bus trip back to the ship.  For anyone interested in Danish history and the wonderful countryside, I would highly recommend this particular tour of North Zealand.