With 12 nights, you have enough time to cover Krakow, Warsaw and Gdansk, but do your 12 nights include arrival and departure days.? While we spent more than a week in each and were definitely not bored, there are plenty of interesting sites in all three but you'll have to narrow your choices considerably. Someone mentioned Wroclaw. I agree that it's cute and fun, but lacks any serious museums, if that's your thing. It really depends on your interests whether or not to include it. I'd give it none two days on a short itinerary just to get a flavor of it, if you decide to go that route, maybe deleting a day from Warsaw and one from either Krakow or Gdansk. That said, we had a couple of great meals there and really enjoyed just walking around (finding dwarves and enjoying Cathedral Island and the main square). In retrospect, personally, I would have added one more day there as the popular "vacation from our vacation) option, although for us Bydgoszcz and Torun served that purpose, too. In all three/four cities I'd definitely try for the orientation tours with Walkative. They cover a lot of ground in a couple hours and if you haven't chosen your main sites, they could help you narrow your choices in order to return for those spots that most interests. My only regret, other than choosing to spend a short time (for us) in Wroclaw, was that we never made it to the countryside. Being of Polish ancestry, I really wanted to get to my grandfather's home town, but several hours by dirt roads didn't thrill my husband, so we skipped it. The emigration museum in Gdansk made up for it for it (along with the ethnographic museum in Krakow), but when I return I'd make the trek. The countryside intrigues me. With your proposed (and if you add Wroclaw) definitely you won't need a car. The trains are easy and cheap. I'd be sure to select two forward facing seats, not the two sets of three that face each other. They are pretty tight, especially if your seatmates are large young men! You can't go wrong with anything you'd choose. Enjoy your trip. We all know you will!!