Definitely go see the bison while you're in Bialowieza. It's one of the few places you can actually see a live European bison in the wild. They live in groups on the Polish side of the border in the primeval forest. You can see them in the Bialowieza National Park, but you must be accompanied by a guide or ranger. There is a variety of other wildlife you'll find in the park including wolves, deer, and wild boar. It's a great experience that you have to have. The park is open during peak season (April to October) from 9am until 5pm everyday. During the offseason you can visit the park between Tuesday and Sunday from 8am to 4pm.