I loved Prague the first time I visited, so I visited Czech Republic again a few years ago and Poland on the same trip. I visited Gdansk, Torun, Wroclaw, and Krakow, all by train (skipped Warsaw) then trained over to Olomouc, which Rick gushed about in his PBS episode on Czech Republic a few years ago as sort of an untouristy Prague. But when I was there (mid-September), Olomouc was completely dead - one of those places where you can't tell if the restaurants are open in the main square until you walk up to them, because there are no people around. I showed up at noon at the TI for the supposedly daily walking tour, but they didn't offer it because there weren't enough other tourists. I guess September isn't the best time to visit!
I suppose a night to check out Olomouc would have been OK but I made the mistake of two nights there. The big squares are pretty nice, and it sure was the not-touristy opposite of Prague, but I still didn't feel the need to spend much time there, whereas I hated to leave Prague both times I visited. Olomouc is worth a stop on the way to somewhere else but doesn't seem worthy as its own destination the way Prague is. I suppose you could do what I did after Olomouc: rent a car and drive west through Moravia to Southern Bohemia and stop in some of the cute towns like Telc and others closer to Cesky Krumlov. There are some nice little towns for sure, but I didn't quite fall in love with the area enough to want to return.
I loved the bigger towns I visited in Poland, however. The Polish countryside isn't so pretty, but the cities are lovely, at least in parts. Krakow is the most authentic of the towns I mentioned, because it was one of the few towns to escape serious bombing in World War II. But I still loved Wroclaw and Gdansk as well. They definitely have some communist-era warts not just charming old squares and buildings, but that's part of the appeal of eastern Europe, I think.
I'm considering a return trip to Krakow but heading east this time to L'Viv in Ukraine. I hear good things. I'd probably head on to Kiev just because I'd feel like I should. Might find a few other towns in Poland to stop in on the way.