As u know (if y have read my posts on other threads) -- I think NAXOS merits 4 days, especially in May. Santorini is only 12 miles long; Naxos is largest (AND most green, most fertile) Cyclades isle ... with Just 2 nights (1.5 days?), you won't get beyond its fab port town. You want "less touristy" spots -- yet Santorini is MOST touristy place in all Greece! (Even waiters aren't greek, they're Bulgarian!) By contrast, on Naxos drive 15 minutes inland in Early May & you can end up as we did, doing things like (1) sitting 40 minutes on a farmer's fence watching him shear sheep with a hand clipper, chatting merrilly despite no language in common (2) having coffee in tree-shaded square of hill village Filoti & kibitzing on domino game between local priest and mustachio'd retiree. (3)discovering an entire mountainside of blinding-white marble -- (4) then finding a marble-cutting plant with tiles sprinkled all around outside. BTW, u know those mysterious pre-Minoan marble statuettes that revolutionized the art of Picasso, Brancusi. Modigliani, Moore etc? NY's Met Museum devotes a whole room to display just 6 -- the tiny Naxian Museum has EIGHT CASES full... from 2" to 3 feet. So, Naxos is worth 4 nights -- & if you want there are day-excursions to Paros, Antiparos etc.
For those other extra days ... why rush away from Olympia ??? Even without a car I found my way south (2 busses & a taxi) on the West Coast. This map - http://euro-map.com/karty-grecii/peloponnes/podrobnaya-turisticheskaya-karta-peloponnesa.jpg (click & it gets huuuuge) shows the route. Stop at NESTOR'S CASTLE (remember him? from the Odyssey?) say hi to 2 Univ of Cincy archeologists who head this dig. Then mosey south to my Secret Eden, a favorite undiscovered gem (shhh! tell no one!) ... Just N. of Pylos is GIALOVA a tiny village 4 blocks long....Stay at ZOE's http://www.zoeresort.com/index.html -- despite the name, an adorable small family hotel. Rooms with canopied balcony overlook aquamarine sea; in May, you can walk 5 miles up this beach almost alone! Nearby • Navoroni bay, site of 2 famed sea victories, ancient modern • Vodokilia Beach one of worlds most stunning (and protected) beaches • wonderful wildllife sanctuary. •3-4 Castles Moving on, E of Pylos you can detour 15 miles to ANCIENT MESSINE, has 2nd longest wall in the WORLD (next to China's). Stunning! NO tourists! ... I URGE you to see this area NOW because it can't hold out long against Super-Luxe World ... one extra-elite hotel already at N. End of that beach. Heading east from there u can stop a few hours at MYSTRA, deserted wonder-town of Medieval buildings. You can do all that in 3-4 days... you'll probably never come this way again, and I sadly predict it will be changed in a few years. Go NOW. Naples & Amalfi easy to do another time, & no need to hurry; they're already totally touricized.
I'm surprised u haven't yet discovered highlights like above. Have u done all this planning without a guidebook?? Alas, the internet alone cannot provide the insight to make the best choices (Ive been to Greece 12x & STILL seek out new books. Browse your Portand Library & take home an armful. but if you want a single reference, I favor ROUGH GUIDE to GREECE. Not a big outlay; in fact Amazon offers used copies of 2014 edition which I like a bit better than the newest edition -- you're not using it for hotel & restaurant info, but for landmarks & culture, and those don't change much. & You don't have to take Big Fat Book Along. Just pull out vital sections (Athens, Cyclades, Pelops, Phrase list) & bind their "spines" w Duct tape and Voila! pocket guide-ettes!