Warnemunde -- My wife and I booked the NCL excursion "Top Ten Sites in Berlin" because we preferred a train ride over a bus and we wanted to make sure we would get back to the ship on time. It was a long day -- 3 hr train ride each way and 8 hrs on a bus touring Berlin. Although an expensive excursion, it was a good trip and we saw most all of the tourist sites in Berlin. It included a nice lunch in Berlin and drinks/snacks on the train. Our son and his friend stayed in Warnemunde and walked to the town and the beach and enjoyed a relaxing day.
Tallinn -- We walked to the old walled town which was on a hill visible from the ship. It took 30 minutes to walk each way and we spend several hours walking around the churches, shops, and restaurants. We were lucky the weather was nice -- 60's and sunny. Tallinn takes euros, as does Germany and Finland. I picked up a walking tour map from the ship's passenger service desk which helped explain the sites.
St Petersburg -- Definitely the highlight of the trip and a gorgeous city! Again lucky on weather -- sunny and temps in high 60's. We booked a two-day "moderate" tour with SPB Tours -- I read about them in earlier reviews and booked the tour online. They are a better value for the money in my opinion -- we would have used them in Berlin except we wanted the train. If you want a tour at every port, they also offer a multi-city package for a good price, but we wanted to go on our own at the other stops. The two days in St Pete were lengthy but outstanding -- we saw all the major sites. The city is beautiful and you will want to spend both days sightseeing. Russia uses rubles but we got by with credit cards for lunch and shopping and tipped with dollars or euros.
Helsinki -- The weather turned cooler, in 50's, with clouds and wind but no rain. Helsinki was okay but a small port. We chose to walk into town but grabbed a taxi since it was windy. We used taxis to go to Rock Church, then to Parliament Square, about 10 euros each time. We walked back to the ship thru the shopping district. Another go-it-yourself option is the Hop On-Hop Off bus which is right by the ship but it cost 25 euros each. If it was raining we would have used it.
Stockholm -- Poured rain during the short 7 hr stay in Stockholm. We booked an NCL excursion to the Vasa Ship & city sightseeing so we did not have to walk around in the rain. Stockholm is a beautiful city and the short stay was not enough to cover the sights. The reason the stop is short is because it takes 4 hrs to sail up the river to the city from the Baltic. We did have a great view of the estuary when we departed Stockholm for the Baltic -- we stayed in our cabin and took pictures from the balcony of the islands and the resorts. Sweden has their own currency so we used credit cards for purchases.