I havent take the ferry before ... but remember to check the schedule because April I rmb, the ferry is less frequent.
I suppose you need to enter and exit in the same place
if it is milan,
fly into Milan, Train to Croatia Captial, Zagreb ( i guess around 6 hour)
go up to slovenia capital, Ljunbjana, you can finish visit it in a half day
then bus to Bled (you can stay their overnight)
after that train back to Zagreb and then Bus to Split (overnight bus), i rmb it takes 8 hours ...
From Split you can bus to Dubrovnik and ferry to the Islands, e.g. Hvar
Split, Dubrovnik and Hvar, will worth 4 - 5 days at least .
After Dubrovnik, you can take overnight bus to Trieste (italy) which you can train back to Milan ( around 4- 5 hours)
In between Zagreb and Split, there is the pitxxxx national park which you can visit ( but I havent)
There is airport near Durbovnik, which you can take budget airline, not very expensive
there may be routes between Dubrovnik and Milan or Rome, so that you can save one leg of using bus transport and save some time.
I havent take the ferry, so I am not sure about the situation of Rome to Pescara
