If you want to get helpful advice you need to give key info not shared so far:
(1) PEOPLE - how many are in your "group" and do you need to find rooms for them all, and is it a case of 2 adults/1 room, or are there kids and needs for triples or whatever? Advice might differ a lot, depending on whether there are 4 of you ... or 10! And do they all arrive at roughly the same time n Day 2, and what time of day IS that?
(2) MONEY- are you trying for a "shoestring trip" or are you on a comfortable budget? I can't figure out the mixed message of your plans -- you outline a trip that involves a lot of costly (and time-consuming) travel, At the same time, traveling "at the end of June" (which must mean it goes into July), you request rooms at $100-$120 tops -- High Season begins July 1, Santorini is one of the costliest destinations, and Athens in convenient areas is also pricey. Also, if you have to rent a car for some reason, more than 5 people will require a van, costly and hard to book.
(3) SEQUENCE - what's the reason for your itinerary Sequence? Is it because your group is coming from many places and will only rendezvous on arrival at ATH airport? Will you arrive late in Day 1, too late to get transport to your first destination besides Athens? We Greece "vets" usually advise going straight to your FARTHEST destination FIRST, if possible, then work your way back. Athens is hard to negotiate for travel-weary newbies at first, if not on a tour with a leader. If you start elsewhere, it's lower-key, you get "the lay of the land," and when you get to Athens at the end, you're very savvy about how to make the most of that sight-filled bustling capital city.
(4) CHOICES - You don't share WHY you are doing Corfu (maybe you saw that TV series the Durrells in Corfu??) Or Grandma came from there?? I really think it can screw up a 2-week trip cost & time-wise... and you really only have 12 days, not 14. You cannot fly from Corfu to Santorini, you must return to ATH airport and take another plane. Also Crete needs at least 4 days and probably a car rental. Crete is not really an island -- it's a COUNTRY, huge. Unless you are going there just to see Palace of Knossos then jump back on a boat. And, in a short 2 weeks, I'd think hard about a day for Sounion... you don't have all that time to spare. it's OK to be newbies, but newbies need to do some of their own research -- and in a GOOD guide book, such as Rough GUide to GReece (free at any library), not just clicking on wonderful photos on the internet -- or you'll end up with unworkable scenarios.
(5) INTERESTS - No indication of whether you are all retired teachers and avid ruins-buffs, or young couples out for sun and swims, or hikers or wine-fanciers, or just scenery-viewers. No point in aiming your group at ancient sites if they scorn old stones, or at landscapes if they'd rather sunbathe; we help better when you share your interests.
POSSIBLE OPTIONS -- But cannot figure how do-able they'd be, depending on the not-disclosed facts about arrival times, # of people, budget limits etc. Here are a couple ideas:
• Fly direct on arrival day to CHANIA Crete (2 days in area, 2 days driving) - Ferry from Heraklion to SANTORINI (2 - 3 days) - ferry NAXOS (3 nights) - ferry to ATHENS (2-3 days)
• Fly direct on arrival day to SANTORINI (3 nights 2 days) - Ferry to NAXOS (3 days) - Fly to ATH Airport - drive to NAFPLIO (3 nights) - Drive to DELPHI, (1 night) then Return to ATHENS ( 3 days). NOTE: you could take intercity bus to NAFPLIO, but wouldn't be able to see as much in the area in a short stay.
Those kinds of trips are do-able, I've done them myself, taking along newbies, and on a budget (tho earlier in the season). If these interest you, share your group info, and RS regulars can help you lock in your logistics. With the itineraries you outline you could end up spending 40% of your time in transit, and at high cost.