Get the ferry from Split to Dubrovnik instead of the bus. It leaves stupidly early and only runs twice a week but its great fun sitting on the deck wathching the landscape go by. I was only in Split for one night, wish that I stayed for longer. The hostel owner in Zagreb got me drunk in the morning and I passed out and had to stay another night in the hostel. Good bit of busines from him I suppose.
Also in the summer there is an over night "disco train" that runs from Zagreb to Split that sounds awesome. You should try that out.