Santorini was lovely. dive shop in Perissa were lovely with us. As has been said, life is a bit sparce, but Santorini's dive sites are grand in scale and offer nice scenic dives... besides diving at an active volcano is well unusual
Land sight seeing is cool too, the archaeological site at Acrotiri is well worth a visit. Diving around Athens has not much to offer but learning there might be ok.... Aegean diving centre in Glyfada has been recommended to me before, i used them for a couple of dives, they were v nice and helpful.
hth