Day 1 : Land in brussels at 6 am and spend the day over here.
Day 2 : Take an am flight to Venice and reach Venice by noon. Spend day in Venice
Day 3: Spend day in Venice
Day 4 : Take the am train to Rome and reach Rome by 12noon. Spend day in Rome.
Day 5 : Spend day in Rome
Day 6 : Spend day in Rome.
Day 7: Take am flight to Athens and reach Athens by 8 am. Spend day in Athens
Day 8 : Take am fligh to santorini and reach Santorini by 8 am.
Day 9 : Spend day in Santorini
Day 10 : Spend day in Santorini
Day 11 : Spend day in Santorini
Day 12: Take am flight back to Athens and spend a day in Athens.
Day 13 : Take am flight to frankfurt
Note : The flights in iten (3) and the same cost as the Euro rail in iten (1). So thats not an issue. But just as a note taking so many flights would get tiring right?