I would definitely recommend trips to Auschwitz/Birkenau and the Salt Mine, you can do them together but it would be a long day with lots and lots of walking. We did Auschwitz with a morning pickup from our apartment, so we didn't have to stand in line to get tickets. We were back at about 4pm. We did a similar booked trip to the Salt mine but in the afternoon - pick up at 3.20 and back about 7.30pm. It is more expensive to book like this but it takes all the hassle out of getting there on public transport and queuing for tickets although you can do that. We went in August when it is very busy.
Other places to spend time - the Old Town which is picturesque, including St. Mary's Basilica; Wawel, the castle, which we couldn't get tickets but we looked around the garden and outside of the buildings. We did do an audio tour of the Cathedral which was excellent. We went to the Underground museum which was interesting but lots of screens to read. A food tour is a great idea, it gives opportunity to get inside knowledge of the city, meet other people and eat local food!
The Jewish quarter is worth a visit too, a different vibe to the old town, different food too.
When we were there there were lots of walks available to see different places, I don't know if they are available in the winter months.
I would recommend that you do some research before you go to see the different attractions/what you are interested in.
We also did a 'graffiti' tour which was very different but interesting and showed different parts of the city.
There are lots of listings eg 3 days in Krakow, 5 days in Krakow etc which help to focus on the 'main attractions', it may help to look at them to get the most out of your trip.
I booked some stuff before I went (because things get booked up in the summer months and we had specific times in different cities) but you can book when you get there, I don't know about availability in January.
Have fun!
