Big cities are very tourist-friendly:

Warsaw -> lots of history = lost of museums (Uprising Museum is a must!) , an astronomical amount of points of interests - Old Town (including Royal Castle and Barbican), New Town (with about 6 churches), the whole Royal Route, Wilanów Palace, Warsaw Citadel, so much greenery including Royal Baths Park, Saxon Garden, Ujazdów Park, you will not run out of things to do but it's pretty crowded wherever you go

Wroclaw -> quite famous Market Square and the Old Town, pretty unique Africarium (a part of Wroclaw Zoo), interesting architecture, lots of old buildings since it wasn't completely destroyed during the war, a ton of bridges, the downsides - it gets boring after some time (so perfect for a weekend) and there's not much outside the middle part of the city

Gdansk -> ships, boats, bridges and again the old-fashioned architecture, Old Town, the Long Market, amazing WWII museum, historic Westerplatte, churches, cathedrals and more churches, personally there's not THAT much so I'd also recommend visiting nearby cities - Sopot and Gdynia, it's like 10min by train

Krakow -> (will fill this out later;)