We are doing the Breakaway as well August 2018, but the itinerary out of Copenhagen! I've also joined CC to see if I can get some insight.
We're doing this cruise also. I haven't researched at all. I never do ship excursions, but with the 50 dollar per port credit I might make an exception.
This is an amazing cruise! Here's some info about what we did - everything was terrific! We didn't do any tours through the cruise line.
In St. Petersburg we went with TJ Travel. They were excellent. I think there were 12 of us in a nice Mercedes van. We were so fortunate to have 3 days in St. Petersburg. In addition to going with them during the day, they arranged for us to have a home-cooked dinner with a Russian family. We were able to bring our daughter (7 yr old) with us and the family had a teenage daughter who hung out with her. We had a fantastic time and I would highly recommend doing that if you have the chance. We booked early and there were so many people from our roll call going with TJ that we got a super price. Great local lunches included. You can tweak the tour if you want (we added in a stop at a fascinating subway station). Just a reminder...you must book with a tour company OR get your own visa for Russia.
TJ offers discounts if you book multiple cities with them. It wasn't why we booked with them for a second city but we loved our time in Warnemunde-Rostock-Schwerin. Berlin was just too long of a day for us so we went to Rostock/Schwerin by train. We loved exploring both cities including the castle, had another great lunch, and had some time to explore Warnemunde before we walked back to the ship.
In Tallinn we did a walking tour.
http://www.traveller.ee/tour/best-tallinn-walking
In Helsinki we wanted to do the Happy Guide Helsinki walking tour but the times didn't work out with our time in port (which wasn't too long). We ended up taking a local bus into town - going to a couple of cathedrals, market, riding the subway, having something to eat.
We started our trip in Stockholm so we had some time to explore. We did a "free" city walking tour which was a great way to get aquainted with the city. Love the old town. We also went to the Vasa Museum and took the Under the Bridges and Royal Canal Tour. I really wish we had more time to spend since there was so much to do.
We disembarked in Copenhagen so we stayed overnight. I highly recommend spending at least a day pre-cruise and a day post-cruise in whatever city you embark/disembark from. In Copenhagen we stayed at the Scandic Front which was in a fantastic location. We did a private walking tour with a group from our roll call. On our own we did a lot of walking and exploring and a boat ride.
We were in a couple other ports but I don't think you'll be going there. If you need any other info, let me know.
It's definitely going to be hard to top this cruise but I am SO looking forward to our Jade cruise...lots of new ports for us! Have fun planning!