Several of the places on your list are tourist destinations in their own right, so there's no need to go beyond them (especially as time would obviously be tight).
The only place I've actually visited is Copenhagen. It's a very compact city centre (and flat), so it's easy to explore on foot (even for fit octogenarians!) or on public transport. There's loads of information here:
http://www.visitcopenhagen.com/copenhagen-tourist
Information about guided tours in the city can be found here:
http://www.visitcopenhagen.com/copenhagen/culture/guided-tours
However one excursion I would suggest is to take the train to Malmo, in Sweden. The trains are frequent (every 20 minutes) and cheap, taking around half an hour. They cross the Oresund Bridge, which is actually a tunnel and a bridge combined. (They meet part way across, on a man-made island).
http://www.malmotown.com/en/
I'd love to go to St Petersburg, if only to see the world-famous Hermitage museum:
http://www.hermitagemuseum.org/html_En/index.html
Like the Louvre or the Prado, it must merit
at least half a day there.
Other ideas can be found here:
http://www.saint-petersburg.com/
Tallinn is definitely on my 'to do' list. Here are some recommendations, from the official website, for visitors with limited time:
http://www.tourism.tallinn.ee/eng/fpage/ideas/must_see
This might also be of interest:
http://www.tourism.tallinn.ee/eng/fpage/explore/sightseeing
In Warnemunde, this tour company
http://www.friendsofdavetours.com/
has excellent reviews
http://www.tripadvisor.co.uk/Attraction_Review-g187364-d1647222-Reviews-Friends_of_Dave_Tours-Warnemunde_Rostock_Mecklenburg_West_Pomerania.html
(Perhaps it's the one used by the cruise company anyway? Even so, it might be cheaper to book direct).
These links might also be of interest:
http://www.visitsaaremaa.ee/index.php?act=atraktsioon&lang=en
http://www.visitaalborg.com/ln-int/aalborg/tourist
http://www.visitkarlskrona.se/en
Chris