Spending eight days in Poland. Can all 3 cities be done in that time? Can I do a one day trip to Gdansk from Warsaw? Thanks for any help!
Yes, it can be done in 8 days, but it will be rushed. Gdańsk deserves more than a day trip from Warsaw but it's doable, too. The fastest train takes about 2hr 50min (one way) - you can check the times at http://rozklad-pkp.pl/en
Stations: Warszawa Centralna, Gdańsk Główny, tickets can be bought from 30 days before travel.
You can also fly with LOT or Ryanair from Chopin Aiport.
That was my trip to Poland exactly in May 2015--one of my favorites, too, visited Poland on a bit of a whim and fell totally in love with it's charm, beauty, food (loved the food!) and the wonderful people. 3 days each in Krakow and Gdansk and 1 in Gdansk is a perfect introduction. Here's my report: https://community.ricksteves.com/travel-forum/trip-reports/poland-prague-part-1-poland
I arrived in Krakow mid-afternoon on a Sunday and took the train to Warsaw early Thursday morning, flew to Prague on Monday afternoon. From Warsaw on Saturday I took a very early train (6:30 am, had me in Gdansk by 9:30) and returned on the 5:30 train--it was a terribly cold and rainy day otherwise I would have stayed a couple of hours longer, as it is I thoroughly enjoyed my 8 hours there and that included exploring the old town, taking the boat out to see the Westerplatte, nice lunch at a restaurant, a bit of souvenir shopping, a visit to the Solidarity Museum (highly recommend) and a quick bite at the café before getting on the train. My tickets to Warsaw and to/from Gdansk were quite cheap, I recall maybe $15-20 US at the time.
Krakow is very compact and walkable, really loved exploring the old town and Kazimierz area, Schindler factory was a must as was a day trip to Auschwitz, Castle Hill good for a few hours to half day. The Cloth Hall has an interesting art museum upstairs, and on the square in the good weather there's always craft and food stands--best meal was my first one of grilled kielbasa and potatoes with an enormous beer.
Warsaw also impressed me, despite being almost totally devastated in WWII it was painstakingly rebuilt and is quite charming. I particularly enjoyed their National Gallery and the Museum of the History of Polish Jews and just spending time along the Vistula and in the old and new towns, lots to see.