There are good open air beach bars in Hvar. Also on the island of Pag at Zrce beach there are some clubs right on the beach and lots of partying. Very young crowd. This is of course only if you go during summer. Pag has very interesting scenery, Dry and Arid but still beautiful and very different that Hvar.
I also recommend Krka national park as it is closer and very nice. You can swim in the waterfall (only in summer). Split is great. Trogir is another beautiful little town onthe coast.