You can take direct fligh to/from Tallin to Warsaw - it's quite cheap with LOT Polish Airlines.
It depends what would you like to see: if you are looking for cities I would recommend (in Poland) Krakow (with Auschwitz concentration camp and Wieliczka or Bochnia solt mine as a must - both Auschwitz and mines might be reached from Krakow by local trains/buses) and Gdansk. If you would like to start from Helsinki you can take a ferry or a plane to Gdansk (very nice 1000 years old city with plenty of historical places where the II world war starter and Solidarity movement + nice Baltic sea + one day in Sopot spa which is in the same city area as Gdansk), then a flight (faster) or train (slower due to rebuilt before Euro 2012) to Warsaw and then flight/train to Krakow