ATHINE - DUBROVNIK
It is very easy to go from Athens to Dubrovnik.
Take the bus from Athens to Patras (every 30 minutes), then a ferry to Bari in Italy (twice daily) and change there to a ferry to Dubrovnik in Croatia (daily in high season). That's all!
There are low cost flights from Athens to Bari too: www.myair.com
Timetables:
Patras - Bari: www.superfast.com, www.agoudimos-lines.com
Bari -Dubrovnik: www.jadrolinija.hr, www.azzurraline.com/ENG, www.dimaiolines
You may also take the daily Athens - Tirana bus. There you change the bus and go up to Shkoder, from there with a minibus to Ulcinj or Bar in Montenegro and catch there the direct bus to Dubrovnik.