absolutely, Rafimondo! Every newbie who goes to the 2 islands says afterwards, I wish I'd spent less time in Santorini & more on Naxos! Maybe 30 years ago Santorini could be enjoyed at length... I was there first 20 years ago & it was OK for 2 nights (Hve been back a number of times but just because I had newbies along for whom it was a must). Now it's SO crowded & tourist-drowned that u just want to view the caldera, and then flee.
I also agree that you can make the most of your time if you don't spllit up Athens... you waste precious hours in the process of going IN to hte city checking into hotel, then checking out etc back out to airport ... waste at least half a day or more. ASAP after arrival (allow 2 hrs layover) fly to NAXOS ... in just 40-45 minutes. I just checked July 7 at the 2 airlines serving Naxos --- and Aegean/Olympic doesn't have anything past 10:30 am (sold out of its 2 other flights i guess.. these are small modern turboprops, only 40 psgrs). But Sky Express has a 10:05, a 12:30 & a 16:15 (3:15) ... it will cost u around $120 or more, but since you are celebrating a big promotion go for it!!
My suggestion is: Spend 5-6days in Naxos, then take the 1pm daily BLUE STAR ferry to Santorini (get economy tickets, sit on upper back deck for the view). Spend 2 nights MAX,. and then proceed to Kefalonia (curious as to why u chose that particular isle), then off to sailing in Croatia.
SInce u will be in Hi season on Naxos, I suggest u stay on PLaka Beach (my friends who are also Naxophiles, praise 3 Brothers hotel there) ... busses every half hour take u to the attractions of port town in 20 minutes, and in July they run until at least midnight. You'll want to rent a car 1 day (or more?) to zoom around the island, up to mountain villages etc. The time will fly by. In my 13 greece trips i've visited 25+ islands, but Naxos is my Must each time, new discoveries. Here, views of Naxos delights, by ace photog "stanbr" --
Trip Around Naxos -- http://www.flickr.com/photos/stanbr54/sets/72157634605629689/ -- this shows port town, beaches, dramatic inland landscapes... ruins, the works.
Week in Naxos 2014 -- http://www.flickr.com/photos/stanbr54/sets/72157634605629689/ - this shows you Plaka beach, and Agia Anna Beach (crowded, narrow), and the 3 wee offshore islands u cn do on a day's "beach cruise" -- plus the delightful sundown by the harbor ... a Greek isle that has it all.
Santorini in July can be like TImes Square @ Rush Hour.... (or see pictures of a Tokyo shopping street)... hard to bellieve. I've steered many people on this forum to staying FIROSTEPHANI -- it's on the north fringe of FIra, walkable to FIra sights in 15 mins, but out of the hubbub, and the package tours & cruise crowds don't know its there -- their busses zoom right by! Many lovely FIrostephani hotels right on the rimside with caldera-view balconies (but book NOW), none of the mob scene that is engulfing OIA. You can catch an early bus up to OIA, and see the views & take pix before the cruisers & bus tours arrive and cram the lanes. When they arrive by the Thousands, flee elsewhere (one place they never go is the ruins of Ancient Thira on a high high bluff above the sea on the other side of the island). Also, don't try for sunset in OIA -- here are some views of that - http://tinyurl.com/lnquk6h . Even if you'd book dinner seats at THE most pricey restaurants there (€€€), hordes of people would be standing staring at u and your plate. Instead, find a restaurant in FIra town or Firostephani with great views, and enjoy serene sunsets.