About the shore excursions: do not take the canal-tour in Copenhagen. We did, but where very fortunately we had seen the city on our own that afternoon. Otherwise we would not have seen anything of Copenhagen at all! There is a shuttle from the ship to the town-center: 4 dollars. And visit Tivoli, it's lovely.
Please take in St. Petersburg the excursion: "The imperial legacy of Peter the Great": it's Great!!!!! Or try to hook up with others and take a 'Red October' tour.
In Tallinn: do not bother to book an excursions: you can reach town-hall easily (+- 15 minutes). It's a lovely medieval city, you really want to go!
And altough Princess told us we had to pay with foreign currency, all the countrys accepted dollars and euros, no trouble at all!
I'm also on the 5/25 Star sailing. Bringing any kids along, by chance? My son is hoping there will be a few. Last we heard from Princess there were only about 15 teens and 20 8-12 year olds booked.
I've done a lot of research so far on the ports, etc, so if you have any questions, feel free to ask. We're planning to tour with Den Rus in St. Petersburg, otherwise will probably do most other ports on our own.
When we were in Tallinn, we had reserved a guide, and he took us to a large outdoor plaza with stalls, sort of like a flea market, but new and very nice merchandise. I bought some amber there with very good prices, wide range of merchandise and prices. Well worth our time. There is only so much time you can spend in the old town.