While there is not all that much to see in Helsinki, it is a great port. Loses points for the port being a long way from the city centre.
We took a taxi into the centre (10 euro) and then bought a Helsinki card (33 euro for an adult, 11 euro for 16 and under) which gives free admission into almost all museums, plus a free guide, boat trip to Suomenlina island fortress (a must see UNESCO world heritage sight with free guided tour), and city tour on a bus. This was a great choice and I highly reccomend doing the same. The card gives you so many included things like public transport as well- it's the smart option compared to an NCL shore excursion.
The Norwegian Jewel also visits Stockholm
Wake up extra early to be greeted with fantastic views of the early morning archipelago as you arrive into port. Again, in this port, shore excursions aren't really necessary as navigation is easy and everyone speaks english. The boat docks at 8.00am so try to get a boat trip and guided tour of an archipelago island which leaves at 8.30. It leaves from outside the Grand Hotel.
For further tips of what to do at each port, please do contact me at firstname.lastname@example.org