If you like water, come to the Mazury. When the final glacier departed Poland, it left behind a legacy of a thousand lakes, most of which are interconnected with rivers that make for excellent water-sports. You can push off from Mragowo which also plays host to a country and western music festival, or from Gizycko which sits to the south of Lake Mamry, or from the more aesthetically-pleasing Mikolajki on the Sniardwy.
If you like castles and churches, come to the neighboring region of Warmia. Here you could visit the numerous Gothic brick churches which testify to the centuries-old Prussian presence. An excellent example of the fortified castles which housed the Bishops of Warmia still stands in Lidzbark Warminski. The spiritual home of the bishops was in Frombork, where that famous Polish son Mikolaj Kopernik also lived out the last decades of his life.
Other, less pleasant reminders of the past also exist, one being the eastern-front headquarters of Hitler in Gierloz. This 'Wolf's Lair' was the site of the failed 1944 assassination attempt, and its remains provide a chilling glimpse of the might of Nazi Germany. Stretching a bit farther back in history, the battle grounds of Grunwald outside of Olsztynek still serve as a symbol of resistance to foreign domination: it was here in 1410 that the increasingly ambitious Teutonic Knights were defeated by a mixed army of Poles, Czechs, Tartars, Lithuanians and others in a rather big and rather bloody battle. In Olsztynek itself, you can enjoy an interesting skansen of 18th and 19th century architecture from this region and from Lithuania. Olsztynek is easily reached from Olsztyn, which also offers a glimpse of Kopernik's life: he lived here as a provincial administrator in the 1500's.