and yes, there is enough to do in Kraków for a week or so. We were there for a week last Christmas / New Year's. Loved it. 10/10 will do again.
The Christmas Market in Rynek with the stalls in the Sukiennice is a very good one. A bit more on offer than the ones in München... I felt it was more like the main one in Salzburg, in that it had a bigger variety of stuff. I'm not a big Xmas market shopper but the wares and prices there were different and interesting. Luggage space restrictions meant we had to be selective. I know my wife would've liked to have scored more booty. But anyways... The market was open every day through December 26... when it downsized by half (staying open nonetheless) and they started building an impressive temporary outdoor venue for New Year's Eve. There was a concert on either the 29th or 30th which I felt was a dress rehearsal for the 31st, but I could be wrong about that since it was pretty well-attended by an enthusiastic crowd. Opening times until 20:00 each night (although I feel like the Sukiennice stalls started shutting earlier, maybe around 18:00?) food stands open longer according to http://www.krakow-info.com/1christm.htm ... I tend to recall that we closed the market down more than once... The Glühwein ("Grzaniec Galicyjski") is 7 zł a mug which is reasonable, pierogi in batches of ten (various types, incl some veggie options), garlic soup for 5 zł... and everything was tasty :)
Due to its central location, it's easy to traverse the Xmas Market at least for a little bit every day, also past the 26th... We couldn't get enough of hearing the Hejnał... or the food... or the mulled wine...
Don't recall any difficulties regarding finding open restaurants, even on the 25th, 26th... I feel like some things closed, but we never were hard up for choices as to where to eat. We airbnb'ed which means we grocery shopped for breakfast stuff; in Germany if you're not stocked up by like 14:00 on the 24th, good luck buying victuals until the 27th... But some supermarkets / convenience stores even opened up on those holidays which I honestly didn't expect. So that was a plus for convenience.
Day trip to Wieliczka (loved it)... a day for the castle... An afternoon visit to Massolit Books for coffee and cake... Definitely enough to do. We spent a great week there four months ago and will certainly go back. Enjoy!