I will be entering Poland from Lithuania and will probably go from Suwalki towards Augustow, from there onto Suchowola and Bialystok, then Siedlce and finally toward Lublin.
I was wondering about it last year but lack of consistent cycle route discourage me a little. There is many sites with cycling routes but all of what I saw concerns only area around some city/touristic area. So, you probably have to design your own route. The single most important rule: avoid national roads if possible.
As for fine places/activities, I would consider two things:
1. ride along National Parks
2. for a couple of days break from cycling - nice canoeing near Suwałki/Augustów on Czarna Hańcza or Rospuda
As for first part it would include (from north to south):
a) Wigierski National Park (actually this is the part when you can think of canoeing)
Basically If I were you I would connect those small maps with some path avoiding national roads. Hm.. I'm starting to think to do it myself :), well maybe next summer...