Next day, head to Ustrzyki Górne. Spend several hours there - I'd recommend walking up to the tripoint of Poland-Ukraine-Slovakia
The OP doesn't like hiking Delph - and I was in that area 15 years ago - people reliably inform me that Ustryzki has been commercialised.
My message to the OP is to avoid like the plague. There is nothing on this earth worse than Polish commercialisation, with its fake Skansen restaurants and garlic butter with every plate of processed food.
The ideas about Lublin (and the salt mine) are sound as a pound:)
Yes Chemikiem - but the OP is in the wrong part of Poland for gallivanting up to Malbork. There are amazing castles everywhere in the South, and please excuse the pun, but when you've seen one pair........castles don't usually go in pairs of course.......but if the OP wanted to see Malbork they would have flown into Gdansk.
All that way to see Malbork........ They will marvel at the modernised factory in the town, at the communist blocks, at the town itself (a slum of a town today sadly after war destruction), and probably wish they hadn't made the long journey.
Gdansk and Krakow are kind of of the the same ilk, so culture vulture though they may be, the double -header doesn't make sense. If they went to Malbork, they might as well go the whole hog and visit Gdansk. And what for? As Delph says, they would be cream-knackered by excess travel. They've already seen remnants of medieval Europe in Krakow and out East (and "better", as you quite rightly intimate)..
PS. If the OP is American she might think Poland end to end is like a trip to the next County........ Here to help:)