If you are staying in Split it is worth taking the 60 bus out to Omiš for the fortress. The bus ride will take you a little over 30 minutes from Split. It is an easy walk/climb to the top of the fortress and it offers great vies of the area. The cost is 15Kuna per person. After you are done with the fortress head down to a local cafe for a beer and enjoy the views of the mountains.