we have been to Krakow and Warsaw before so somewhere new would be nice
Wroclaw is wonderful. Poznan is pretty nice. Lublin is much smaller but still well worth visiting (although the road there is pretty rubbish). Zamosc is tiny and a pain to get to but superb. Gdansk/Tri-city is pretty good but a long way from anywhere else you might want to visit (apart maybe from Torun).
For two specific and very different destinations, check out Reszel and Arlamow.
We will be driving around
Personally, I try to stay off the roads in February. The major routes will be OK, but a cross-country journey can easily turn into a nightmare. Also, if you're visiting major cities, you can expect to pay more in parking charges than you'd pay to a train ticket (for example, my favourite hotel in Wroclaw charges 60zl per day in parking but a ticket on the Pendolino will set you back from 49zl).