As already suggested, you could plot a visit to Auschwitz during your stay in Kraków, touching but definitely worth it. You could also perhaps spend an extra day in Trójmiasto (i.e. Gdańsk, Sopot & Gdynia), a few days on the seaside instead of Warszawa/Kraków (where the absurd level of air pollution is probably only second to Beijing) could kick things off nicely for you (unless it starts snowing like yesterday). As for transport, you could also look at flights, with Ryanair often offering ridiculously low fares (~€5) between Gdańsk & Warszawa, for instance, otherwise trains (relatively cheap for western standards and with the big plus being the high speed rail service, introduced back in 2014), but I wouldn't bother with coaches (cheap, true, but road works, traffic etc. would personally put me off). There are countless things you could see in the above mentioned places, though if you could be a tad more specific we would surely come up with a decent list.
G