I toured the Transylvania and the more north eastern Suceava/Bucuvina areas. I highly recommend the painted churches up there along with the hilly, countryside views, it was great. I also focused my tour on the Vlad Tepes/Dracula legends so kind of nerdy. I went to see his birth house in Sighisoara, his castle at Bran (not really but a basis for Bram Stoker), the burial place of his headless body in Snagov, just north of Bucharest, etc. If you are not a dork, I still recommend Sighisoara, Sibiu, Brasov right in the center of Transylvania. It might be better to have a couple of days if you go north to Suceava. I had a private driver as part of a personalized tour since I was not comfortable driving in a foreign country and trying to see the sights. It worked well for me but isn't super cheap. I've ridden 1 train from the east part of Romania (Bacau) to Brasov and the busses are much better and nicer. Since you are going north, you might want to hit Cluj or the wooden churches in Maramures (neither of which I have visited yet).

I'm going to try and link to my facebook album. I travelled in May of 2013 so the weather and views should be very similar for you Sadly, I can not help with Slovakia or Poland.