So many beautifull places to visit...
My recommendation is, don't hesitate to try off the beaten path.
Islands like syros, folegandros etc. or mainlland like Metsovo, Papigo, Dimitsana, Arachova (even in the summer)
Some great tourist spots are Monemvasia, Naxos ok Santorini is the best :) (expensive and lots of people in midseason but still amazing) soo many :)