Day trip to Malbork Castle from Gdansk
As the site of the largest castle in the world (unsurprisingly a World Heritage site), it is likely that you will feel the pull to visit Malbork, 30 minutes by train from Gdansk. The castle is a 17-minute walk from the train station, but if you're following my itinerary at all, I doubt this will pose an issue for you. You're about to spend 3-4 hours wandering around a castle, so it isn't terribly significant anyway.
Getting to the Castle
Buy your tickets ahead of time here, as this will significantly reduce how long you have to wait around for before starting. There may still be a line to enter the paid section of the castle a few minute's walk from where you pick up your audio guide, as there was when I left the castle, but if you start your tour early enough in the morning, this should not be an issue.
Visiting this castle was one of my favourite things I did in Poland. I live in Luxembourg, home to Vianden Castle and the Valley of 7 Castles, and I've visited plenty in Ireland, Germany, and Belgium, so I consider myself to be pretty "castled-out" in general. Malbork Castle is a fully restored Gothic-style castle that was heavily damaged in World War 2. It houses extensive exhibits on its history as the seat of the Teutonic Order of Knights and a monastery in addition to containing a wonderful armoury (everyone loves the shiny pointy things, assorted other weaponry, and armour, right?) and a breathtaking amber museum featuring some incredible amber artwork.
I've found that the enjoyment of a castle visit largely depends on the quality of the audioguide, or, if unavailable, the quality of the exhibition material. The audioguide here is fantastic and based on hotspots, meaning that the relevant information starts playing as soon as you walk into an area. My tip is to skip the "multimedia presentations" viewable through eye-holes if it is crowded. There are a number of times the guide tells you to continue after viewing the presentation, but they are just clips of actors recreating medieval scenes. Continue on immediately if there is a line, there is so much in this place far worthier of your time.
There is a restaurant inside the castle that I was forced to have lunch at because I didn't bring something for myself, and it is a long visit. It was okay (though extremely slow), but if I went again, I'd just bring a sandwich and eat it in the massive courtyard. Take a long break if you're feeling overwhelmed with information, it's a big place and there's a lot to see.