The number one activity here, at least in summer, is hiking. Ustrzyki Górne is the most popular base for hiking the Bieszczady National Park and several great walks along colour-coded trails start out from here (most from near the camping ground or the bus station and car park).
You'll find several longer and shorter routes marked out on hiking maps, but one popular loop to get you started begins in the village of Wołosate (take the regular bus from Ustrzyki Górne, about 3zł, 15 minutes). From here, follow the blue path (two hours) to the region's highest peak at Tarnica (1346m) and then back to Ustrzyki Górne along the red path (two to three hours).
The park is full of fascinating sights and sounds: the remnants of villages abandoned or destroyed during Operation Vistula, ancient cemeteries, peat reserves (complete with quicksand) and the cry of a lone wolf in the distance.