Quite a wide ranging question. All except St Petersburg can be explored independently. Unless you speak Russian, either a ship's tour or Alla or Denrus etc would be best.
Stockholm - depends where you are berthed, but could be walkable. Old town worth a visit but Vasa Museum a must. You can get lunch here without breaking the bank.
Helsinki - nice city. Lovely shops including Stockmann's (Finland's Macey's). Can take a round trip on a tram.
Tallin - beautiful, try to get to the old town early before the crowds.
Poland good for shopping particularly glass ware.
Warnemunde - not keen myself, but nice beach if the weather's good.
Copenhagen - busy but worth the effort. Walkable into town along the harbour side. Harbour cruise is pleasent and informative. Bishop's Palace worth a visit. Tivoli Gardens a bit of a rip off, but you should probably go anyway. Little mermaid usually covered in Japanese tourists.
Have a good holiday.
Are there any taxi's at the ports or do you have to walk if you don't go on the ships organised trips.
Normally a queue of taxis just outside the dock gates. Most cruise lines arrange a shuttle bus into town, but these days they charge anything from £2 upwards.
Very helpful, thanks.