Senj, dubbed "the fortress city," has two dubious distinctions: First, it has a reputation as the coldest, windiest town in Croatia; and second, it is the site of an infamous 16th-century pirates' den that was home to the flamboyant Uskoks, who lived in Senj in the 16th century and preyed on passing merchant ships.
In 1535, the Uskoks were a community of controversial anti-Turk fighters who helped fend off Ottoman assaults on the city and later channeled their aggression toward more lucrative pursuits until they were driven out in the early 17th century.
Today's Senj is worth a stop to see Nehaj Fortress, which was built by the Uskoks and which makes a great photo op from the sea or highway and a great perch For photos of the sea and skyline. The Senj Town Museum is also worth a quick look to get a sense of Senj's history, but neither is worth a lot of time.
South of Senj, you can catch the ferry to Pag Island at Prizna or a ferry to Rab from Jablanac, but on land there aren't many settlements of note between Senj and Zadar -- and the ones that are there don't have much to recommend them. In fact, Zadar is the next major population center on the coast south of Senj.
Those who prefer uninhabited, unspoiled, rugged terrain to city development can veer off the coastal highway to the Velebit Range, which includes Paklenica National Park south of Senj. It extends almost the entire length of the coast from Senj to Zadar.